Check availability button in purchase requisition

Question

Dear All,
What is the use of the "Check availability" button provided above the item over view sub screen of the ME51N transaction.This button one among the other buttons provided above the Item over view sub screen.
To enable this we need to do any customisation.
Please share your ideas for which iam very much thankful.
Advance thanks,
Regards,
Dayanand

Answer 1

There are two types of availability check:
_ Availability check for the various stock types in Inventory Management
_ Check for the available stocks in Materials Planning (MRP)
Availability check of the stock types is carried out automatically and cannot be manually set in the
system. Non-availability results in an error message.
The available stocks check in MRP is activated in the material master record and can be
configured in Customizing for MRP (refer to Net Requirements Calculation [Ext.]).
Your system administrator can define:
_ Whether an availability check is to be carried out
_ Whether non-availability is to result in a warning message or an error message
In Customizing for Inventory Management, you can configure the dynamic availability
check for goods movements. In the step Define Checking Rule, you determine for
each checking rule which stocks, receipts, and issues should be included in the
availability overview (transaction code CO09).
Remember that the system does not update the consumption statistics in the material master
record if you post goods movements using the special stock types Sales order stock and Project
stock.
For every material movement, the system automatically performs an availability check of the
stock types if this has been defined for a material. The availability check prevents the book
inventory balance of the various physical stock types (for example, unrestricted-use stock) from
becoming negative.
If several withdrawals of material are entered in a single document (for example, for different
account assignments), the system checks the availability of the material for each item entered. It
checks whether the desired quantity can be withdrawn. The system takes into account those
items of the document that have already been entered, but not yet posted.
If there is not enough stock to cover the withdrawal, the system issues an error message.
The availability check includes the stock in question at plant, storage-location, and special-stocks
level, for the current month and, if necessary, for the previous month. Different stocks are
checked, depending on the movement type:
_ In the case of a withdrawal for consumption, unrestricted-use stock is checked.
_ In the case of a release from stock in quality inspection, quality inspection stock is
checked.
_ In the case of consumption of consignment material, consignment stock for unrestricted
use is checked.
In addition to checking the various types of physical stock, you can also check available stock
from a Materials Planning perspective. With this type of availability check, you can, for example,
prevent a quantity already reserved from being used for another purpose.
There are 100 pieces of a material in unrestricted-use stock. 70 pieces of these 100
have been reserved for a particular requirement date.
You want to withdraw 50 pieces for a different purpose. If the availability check is
active, depending on how the system is set, you will receive:
_ A warning message
_ An error message
_ No message