A Short Tutorial on User Exits

User exits :
1. Introduction
2. How to find user exits
3. Using Project management of SAP Enhancements 
Content Author: Abhishek

1. Introduction:
User exits (Function module exits) are exits developed by SAP. The exit is implementerd as a call to a functionmodule. The code for the function module is writeen by the developer. You are not writing the code directly in the function module, but in the include that is implemented in the function module.
The naming standard of function modules for functionmodule exits is: 
EXIT_<program name><3 digit suffix>
The call to a functionmodule exit is implemented as:
CALL CUSTOMER.-FUNCTION <3 digit suffix>

Example:
The program for transaction VA01 Create salesorder is SAPMV45A
If you search for CALL CUSTOMER-FUNCTION i program
SAPMV45A you will find ( Among other user exits):
CALL CUSTOMER-FUNCTION '003'
  exporting
    xvbak   = vbak
    xvbuk   = vbuk
    xkomk   = tkomk
  importing
    lvf_subrc = lvf_subrc
  tables
    xvbfa = xvbfa
    xvbap = xvbap
    xvbup = xvbup.
The exit calls function module EXIT_SAPMV45A_003

2. How to find user exits?

Display the program where you are searching for and exit and search for CALL CUSTOMER-EXIT
If you know the Exit name, go to transaction CMOD.
Choose menu Utillities->SAP Enhancements. Enter the exit name and press enter.
You will now come to a screen that shows the function module exits for the exit.

3. Using Project management of SAP Enhancements, we want to create a project to enahance trasnaction

VA01 .
- Go to transaction CMOD
- Create a project called ZVA01
- Choose the Enhancement assign radio button and press the Change button
In the first column enter V45A0002 Predefine sold-to party in sales document.
Note that an enhancement can only be used in 1 project. If the enhancement is already in use, and error message will be displayed
Press Save

Press Components. You can now see that enhancement uses user exit EXIT_SAPMV45A_002. Double click on the exit.
Now the function module is displayed. Double click on include ZXVVAU04 in the function module
Insert the following code into the include: E_KUNNR = '1010000000'.
Activate the include program. Go back to CMOD and activate the project.
Goto transaction VA01 and craete a salesorder.
Note that Sold-to-party now automatically is "1010000000"