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The following document is about exits in SAP :-

The R/3 enhancement concept allows you to add your own functionality to SAP’s standard business applications without having to modify the original applications. 

SAP creates user 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.

Types of Exits 
There are several different types of user exits. Each of these exits acts as hooks where you can attach or "hang" your own add-ons.

Menu Exits
Menu exits add items to the pulldown menus in standard SAP applications. You can use these menu items to call up your own screens or to trigger entire add-on applications.

SAP creates menu exits by defining special menu items in the Menu Painter. These special entries have function codes that begin with "+" (a plus sign). You specify the menu item’s text when activating the item in an add-on project.

Screen Exits
Screen exits add fields to screens in R/3 applications. SAP creates screen exits by placing special subscreen areas on a standard R/3 screen and calling a customer subscreen from the standard screen’s flow logic.

Function Module Exits 
Function module exits add functions to R/3 applications. Function module exits play a role in both menu and screen exits. 

When you add a new menu item to a standard pull down menu, you use a function module exit to define the actions that should take place once your menu is activated. 

Function module exits also control the data flow between standard programs and screen exit fields. SAP application developers create function module exits by writing calls to customer functions into the source code of standard R/3 programs. 

These calls have the following syntax: 

CALL CUSTOMER-FUNCTION ‘001’.

Field Exits
Field exits allow you to create your own programming logic for any data element in the Dictionary. You can use this logic to carry out checks, conversions, or business-related processing for any screen field.  Example: The data element BBBNR identifies a company’s international location number. You might want to set up your R/3 System so that all international location numbers are larger than 100. 

The field exit concept lets you create a special function module that contains this logic. 

You assign the special function module to the data element BBBNR. You then assign the module to any programs and screens in which users can add new international location numbers. When you activate your field exit, the system automatically triggers your special routine whenever a user enters a company location number. 

In 4.6c, you can use "RSMODPRF" program to create field exits.

An example of a user exits :-

MODULE user_exit_0001 INPUT 
    CASE okcode.
        WHEN 'BACK OR EXIT'.
            CASE sy-dynnr.
                    WHEN '100'.
                         SET SCREEN 0.
                         LEAVE SCREEN.
                    WHEN '200'.
******************************************************************************
**** Note that you can write any code that satisfy your needs.                                                    ****
**** But in this case, this was wrote as a sample code for reference sake.                                   ****
**** And you can test it.                                                                                                           ****
****************************************************************************** 
                         SET SCREEN 100.
                         LEAVE SCREEN.
             ENDCASE.
      ENDCASE.