Regarding Enhancements/User-Exits in ABAP

Question

Hi,
Can anybody tell me What is meant BY Enhancements & User-Exits.
Also what are diffrent types of Enhancements/User-Exits avialable.
Can anybody explain me about diffrent types of Enhancements/User-Exits.
Can anybody provide me documentation with the examples
for diffrent types of Enhancements/User-Exits.
If anybody is having good material on the same please post it.
Thanks in advanace.
Thanks & Regards,
Rayeez.

Answer 1

Customizing exits allow you to add your own functionality to SAP’s standard business applications without having to modify the original applications. SAP creates customer exits for specific programs, screens, and menus within standard R/3 applications. These exits do not contain any functionality. Instead, the customer exits act as hooks. You can hang your own add-on functionality onto these hooks.
If you want to enhance the functionality of your R/3 System, you should take advantage of the exits available in standard R/3 applications. There are two main reasons why you should use exits rather than modifying SAP software yourself. Add-ons attached to exits have the advantage that:
•     They do not affect standard SAP source code
When you add new functionality to your SAP System using SAP’s exits, you do not alter the source code of standard SAP programs in any way. The code and screens you create are encapsulated as separate objects. These customer objects are linked to standard applications, but exist separately from SAP’s standard software package.
•     They do not affect software updates
When you add new functionality to your R/3 System using SAP’s exits, your objects (called customer objects) must adhere to strict naming conventions. When it comes time to upgrade a to a new software release, customer objects’ names ensure that they will not be affected by any changes or new additions to the standard software package.
Customer exits are not available for all programs and screens found in R/3 standard applications. You can only use customer exits if they already exist in the R/3 System. You find find more information about locating applications with pre-defined exits in Locating Applications that have Exits.
     As part of the enhancement concept, it is possible for the customer to 
     add his own elements to application logic, screens and menus.                                                                               
The current possibilities for enhancement are:                                                                               
Text enhancements:                                                     
     Allow the customer to add supplementary documentation for data fields  
     non-specific to a transaction, and to change key word texts.                                                                               
Field exits:                                                           
     Every screen element with data element reference can branch to PBO or  
     prior to PAI to a function module if desired. The field contents are   
     available here for doing special checks and making changes (e.g. user- 
     specific checks, authority checks, writing entered data and producing  
     statistics...).                                                                               
Function exits:                                                        
     From the main program you branch into a software level, in which you you
     can store ABAP/4 coding. The applications programmer at SAP determines 
     where in the main program the function exit is placed, and which data is
     imported/exported via the interface. The accompanying documentation    
     describes the functionality of the function exit.      
     Menu enhancements:                                                       
     Pre-conceived menu items can be activated and named. On the function code
     set at menu item selection, there can be a reaction in a relevant        
     function exit.                                                                               
Screen enhancements:                                                     
     The customer can determine the layout of areas in screens provided by the
     applications developer. Here, additional information can be displayed or 
     data entered.