27 Custom Function Modules
A custom Function Module in SAP ABAP is a reusable, globally accessible block of code that executes specific business logic and resides within a central function library. It is created via transaction code SE37 (or the ABAP Development Tools in Eclipse environment) and must always begin with the letter Z or Y to comply with customer namespace rules.
Before creating a function module, you must have a Function Group to store it. A function group acts as a logical container and dictates the global data variables shared among its modules.
To run this function module in an executable report program (SE38), use the CALL FUNCTION pattern. Notice that parameters invert: Importing in the module matches Exporting in the caller.