What is availability check concept how it is used in SAP

Question

hi friends,
i don't know what is availity check,how is it used and how it should be configured,please help me.

Answer 1

Availability Check in Sales and Distribution Processing 
There are three types of availability check:
•     Check on the basis of the ATP quantities
•     Check against product allocation
•     Check against planning
In Customizing, you determine whether an availability check is to be carried out against the ATP quanitity or against planning. The check against product allocations is controlled in the material master and elsewhere in the system.
Check on the Basis of the ATP Quantities
The ATP quantity (ATP = Available To Promise) is calculated from the warehouse stock, the planned inward movements of stock (production orders, purchase orders, planned orders) and the planned outward movements of stock (sales orders, deliveries, reservations). This type of check is performed dynamically for each transaction, taking into account the relevant stock and planned goods movements with or without replenishment lead time. Planned independent requirements are not taken into account here.
Check against Product Allocation
Product allocation facilitates period-based distribution of products for certain customers or regions. As of Release 3.0F, you can carry out an availability check against product allocation. This ensures, for example, that when production is low, the first customer does not get the full amount, resulting in following sales orders not being confirmed or being confirmed far too late.
Check against planning
The check against planning is performed against independent requirements which are usually created for an ‘anonymous’ market rather than being customer-specific (for example, in the strategy ‘Planning without assembly’, when production occurs only up to the stocking level). The planned independent requirements result from demand program planning and are used for planning expected sales quantities independent of orders
Controlling the Availability Check in Sales and Distribution Processing 
You control the availability check using general and SD-specific control features.
General Control Features
The following control elements need to be maintained in Customizing and in the material master record:
•     Strategy group
The allowed planning strategies (main strategy and further possible strategies) are combined in the strategy group. As of Release 3.0, the strategy group is specified in the material master record in the MRP 1 Screen. In Customizing, strategy groups are assigned, dependent on plant, to MRP groups. If the strategy group is missing in the material master record, it is determined on the basis of the MRP group, if it has been maintained.
Up to Release 3.0, the strategy group is determined on the basis of the MRP group
•     MRP group
The MRP group combines materials from the point of view of material requirements planning. This enables you to control planning for these materials in a particular way using, for example, the strategy group, consumption mode and planning period. You enter the MRP group in the material master in the MRP 1 screen. The strategy group is determined from the MRP group.
•     Planning Strategy
The planning strategy specifies the requirements type for planning and customer requirements. This represents the decisive control feature for the interaction between Production Planning and Sales and Distribution.
•     MRP type and item category
If no requirements type is found using the planning strategy, the system tries to determine a corresponding requirements type on the basis of the MRP type and the item category.
Until 3.0C, determination of the requirements type via planning strategy has taken priority. This is not always the best option, however, as the following example will show. A requirements type is determined for a material, which causes availability to be checked against planning. In consignment stock processing, however, availability should be checked against stock. Until 3.0C the availability check had to be deactivated in these cases. As of Release 3.0C, you can determine how the requirements type is to be determined for each transaction by maintaining the Source field in Determination of requirement types using transaction in Customizing.
•     Requirements type
The various requirements are identified by their requirements type. The requirements type refers to the requirements class and its control features.
•     Requirements Class
The requirements class contains all control features for planning such as relevance for planning, requirements planning strategy and requirements consumption strategy. In addition, it is specified at a global level whether an availability check is to take place for the material in the sales and distribution documents on the basis of the ATP quantity (ATP = available to promise) and whether requirements are to be passed on. A finer degree of control can be obtained for sales documents using the schedule line category.
Control Features Specific to Sales and Distribution
The following SD-specific control features need to be maintained in Customizing:
•     Checking group
The checking group controls whether the system is to create indivdual or collective requirements in sales and shipping processing. In addition, a material block for the availability check with transfer of requirements can be set here. The checking group can also be used to deactivate the availability check. This option was created especially for the assembly order so that when the bill of material is exploded in the assembly order, the individual components, if necessary, can be classified as non-critical parts as far as procurement is concerned.
The checking group specifies in combination with the checking rule the scope of the availability check. It is proposed in the material master record on the basis of the material type and the plant, and copied into the sales and distribution documents.
•     Checking Rule
You use the checking rule to control the scope of the availability check for each transaction in sales and distribution. You also specify whether the check should be carried out including or excluding replenishment lead time. The individual checking rules define by transaction, which stock and inward and outward movement of goods should be taken into account for the availability check.
•     Schedule line category
You can control with the schedule line category whether an availability check and transfer of requirements should be carried out in the sales documents. The possible settings for this at schedule line level are dependent on the settings in the requirements class which is determined from the requirements type of the material.
•     Delivery item category
The delivery item category can be used to control whether an availability check takes place in deliveries.
Prerequisites
An availability check can only be carried out if the following prerequisites have been fulfilled:
•     The control elements described above for the availability check must be maintained in Customizing for Sales and the relevant assignments made to the sales transactions
•     The availability check must be switched on at requirements class level and - for the availability check in the sales documents - at schedule line category level
•     A requirements type must exist by which the requirements class can be found
•     A plant must be defined. It can either be proposed from the customer or material master record or can be entered manually in the document.
•     A checking group must be defined in the material master record on the Sales/plant data screen in the Availability check field
Timing of the Availability Check in Sales and Distribution Processing 
When you create an order, the system determines the required materail availability date on the basis of the customer’s requested delivery date. On this date, you must begin picking, packing, labeling, and loading the goods. Therefore, this is the date of significance for requirements planning on which the availability should be checked.
The following data is required for determining this date:
•     Route from the shipping point to the ship-to party location
•     Shipping point from which the goods are issued
•     Loading group from the material master record
•     Weight group determined from the order using the order quantity
Scheduling
This data, which you have already entered in the system, means that scheduling can occur automatically. Since scheduling is carried out backwards from the requested delivery date, it is also called backward scheduling. If backward scheduling determines that the preparation for the shipping activities should have been started already to meet the customer's requested delivery date, the system then starts forward scheduling automatically from the current date.
Scope of the Availability Check in Sales and Distribution Processing 
The following elements can be included in the availability check:
•     Stock
o     safety stock
o     stock in transfer
o     quality inspection
o     blocked stock
•     Inward/Outward movement of goods
o     purchase orders
o     purchase requisitions
o     planned orders
o     production orders
o     reservations
o     dependent reservations
o     dependent requirements
o     sales requirements
o     delivery requirements
Requirements in sales and distribution (sales requirements and delivery requirements) result from all transactions which forward a requirement to Materials Management (MM) or to Production Planning (PP). For example, this could include sales orders or deliveries and quotations as well. Sales and distribution requirements reduce existing stock or inward movements of stock on the material availability date to ensure that other outward movement of stock elements cannot access the quantity reserved in this way.
Requirements relevant for Sales and distribution are created in Sales and Distribution, whereas other elements in this list are created in Materials Management or in Production Planning.
Defining the Elements to be Included in Check
A checking rule is assigned to each transaction. This rule in combination with the checking group controls the scope of the availability check. You can use the checking rules in Customizing for Sales to specify for the various transactions which of the elements listed above should be included in the availability check.
For trading goods it does not make sense to include planned or production orders, for example, in the availability check. However, for products manufactured by your company these orders should be included in the check.
For transactions such as make-to-order production, consignment or returnable packaging processing that create special stock, the availability check is performed against special stock.
If it is defined by the checking rules that both sales and delivery requirements are taken into account in the availability check in sales documents but only delivery requirements are taken into account in the availability check in deliveries, there is a danger that quantities reserved in the sales documents are considered to be available by the availability check in the deliveries. This can lead to sales documents becoming backlogged.
Reactions to the Availability Check in Sales Documents 
If the goods ordered by the customer are not available on the requested date, the system branches automatically during sales document processing to a further screen where delivery proposals are offered for selection. The system determines these proposals on the basis of the availability situation. This screen also provides you with information on the scope of the check, the current ATP quantity, and the availability situation across all plants.
Availability Control
On the Availability Control screen, you can choose between the following delivery proposals:
•     One-time delivery on the requested delivery date
In this section, the system checks whether the requested delivery date can be kept to. If stock of the material is available to make a delivery on the requested delivery date, the stock quantity is confirmed here. If there is no stock available, confirmed quantity zero is displayed.
To copy this data into the sales document, select Edit One-time delivery.
•     Complete delivery
In this section, the system checks whether there will be sufficient stock for complete delivery at a later date:
o     If there is sufficient stock available at a later date to cover the required quantity in the sales document, the system proposes the date here.
o     If the system determines that complete delivery cannot be made at a later date, no date is proposed in this section.
When availability is checked including replenishment lead time, the date which is proposed in this section is the date on which the replenishment lead time period ends if the stock before the end of replenishment lead time does not cover the order quantity.
When availability is checked excluding replenishment lead time, the system bases its calculations on the stock and the planned inward movements of stock.
To copy this data into the sales document, select Edit Complete delivery
•     Delivery proposal
In this section, the system checks whether and for which dates partial deliveries can be made. Partial deliveries are displayed for different dates. These dates are based on the planned inward and outward movements of stock.
During an availability check which takes replenishment lead time into account, the date on which replenishment lead time ends is displayed if insufficient stock means that no partial deliveries can be made before replenishment lead time ends.
During an availability check which does not take replenishment lead time into account, the system displays the dates on which partial deliveries can be made with the available stock.
To copy this proposal into the sales document, select Edit Delivery proposal
The dates displayed in the sections complete delivery and delivery proposal are determined from the material availability date using forward scheduling and represent actual delivery dates.
Availability Across Plants
You can display the availability situation in the various plants for which a material is maintained. To do this, proceed as follows:
1.     Select Goto Other plants on the availability control screen.
The dialog box Plant Selection appears. All plants for which the material is maintained are displayed.
2.     Mark the plants in which you want to check availability and select Check plants.
You reach the Availability Check: Overview screen. The requirements quantity and the quantity which could be confirmed as well as some additional information is displayed in a separate line for each plant you selected.
3.     If you require detailed information on the availability check for a particular line, select the relevant line and select Edit Check availability.
You reach the Date proposals dialog box where the detailed data is displayed.
4.     You can select a plant with sufficient quantity for confirmation to be copied automatically into the Overview screen. Select the appropriate line and select Edit Copy plant.
Using the user exit USEREXIT_PLANT_SELECTION in the Include LV03VFZZ you can make a preselection of the allowed plants. If you use this option, the Plant selection dialog box does not appear and the availability check is performed immediately for the allowed plants. If, on the basis of information resulting from the availability check, you want to deliver from a plant other than the one specified in the item, you must at present enter this plant manually in the item. In a later release, however, you will be able to select the plant directly from the list of plants.
Displaying the ATP Quantities
If you want to display the current ATP situation on the availability control screen, select Goto Availability overview.
Displaying the Scope of the Check
If you want to display the scope of the check (stock and inward/outward movements of goods taken into account during the availability check) from the availability control screen, select Goto Scope of check.
Delivery Agreements in the Customer Master Record
A customer may react in different ways to the fact that a complete delivery can not be provided for the required delivery date. You can define in the customer master record, the way a customer reacts to the inability to meet the requirements represented by the key data.
•     The customer wants complete delivery of all items by the requested delivery date or at a later date.
The complete delivery indicator is copied into the sales document from the customer master record. To find it, select Header Business data on an overview screen or a header screen. You find the indicator in the Shipping section.
If the complete delivery indicator is set in the sales document, the individual items are automatically correlated when the document is saved if the schedule lines exist for various dates (that is, a delivery group is automatically created for the sales document items and the latest delivery date within this group becomes the group delivery date). For further information on delivery groups, see  Combining Sales Document Items for Delivery.
•     The customer only allows a certain number of partial deliveries for each item.
The partial delivery indicator and the number of partial deliveries are copied into the sales document item from the customer master record. To find the indicators:
1.     Go to the Business Data screen for this item.
2.     Select Edit Shipping details.
The indicators are found in the Shipping section.
Performing the Availability Check Again
Usually, an availability check is only performed when a sales document is created. If quantities or dates are changed in the document and availability needs to be checked again, the check is performed again automatically . However, you can also initiate an availability check manually.
Automatic Checks
In the following situations, a new availability check is automatically carried out:
•     Quantity change
•     Date change (requested delivery date, material availability date)
•     Change to the fixed data
•     After new delivery scheduling (for example, the route or shipping point has been changed)
•     Change of plant
•     Change of storage location or batch
•     Removal of blocks which affect requirements
•     Removal of reasons for rejection
Initiating a Check Manually
In the sales document, you can initiate a new availability check manually. If you want to perform an availability check for all items in a sales document, select Sales document Check availability on the header screen. If you want to check availability for an individual item, select Edit Item availability.
Fixing Quantities and Dates in Sales Documents 
If the availability check for an item determines that the item cannot be delivered fully on the requested delivery date, and, as a result one or more schedule lines have to be created for later delivery dates, the customer can decide whether to accept these delivery dates and quantities.
You can record the customer's decision either by setting an indicator in the Fixed date and qty field or by leaving this field blank. The field is displayed both on the schedule line overview screen and on the availability screen.
Customer Accepts the Schedule Line Proposal
If the customer accepts the schedule lines which are proposed by the system, mark the Fixed date and qty field. The system then transfers the requirements for the schedule lines to material requirements planning.
Customer Requires Goods Earlier
If the customer requires the goods earlier than the date that the system can confirm after the availability check, leave the Fixed date and qty field blank. The system then only passes on the requirements for the requested delivery date and does not take the quantities and dates confirmed by the system after the availability check into account. In this case, it is assumed that the requested delivery date can be kept to because, for example, additional production capacity may become free. The system only confirms the sales order once the entire order quantity can be delivered on the requested delivery date. In the subsequent delivery processing, the sales order is only considered to be completed when the entire order quantity has been delivered or when all partial delivery agreements have been fulfilled.
A fixed item is considered to be completed once the entire confirmed quantity (which can be smaller than the originally requested order quantity)has been delivered. Therefore, it does not make sense to fix the confirmed quantity to 0 as the item is considered to be completed from the start.
During rescheduling, the fixed quantities and dates are no longer taken into account.
Blocking Confirmation of Quantities in Sales Documents 
In Customizing, using Block transfer of requirements by delivery block, you can specify that certain delivery blocks in the sales document header also block the confirmation of order quantities.
If you enter such a delivery block in a sales order header, the system confirms the available quantity to show you the delivery options. If you save the document, however, the confirmed quantities are not saved. Your company could block confirmation for a customer if this customer's credit worthiness was questionable. The sales order in this case is not only classed as unconfirmed, it is also blocked for delivery.
Thus, no stock is reserved. The stock is still available for other customers. Although the order quantities are not confirmed, the system still transfers the requirements to material requirements planning. It is assumed that the delivery block will be removed at some point. The automatic transfer of requirements means that the order quantity can be confirmed at a later point in time
Source: SAP Library
Hope this is of some help.