There are several different types of customer exits. Each of these exits acts as hooks where you can attach or "hang" your own add-ons.
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 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 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 pulldown 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’.
From Release 4.5A, you can change field texts on screens and add data element documentation using the Modification Assistant.