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 customer exits for specific programs, screens, and menus within standard R/3 applications.
User Exits is slot provided by SAP in SAP standard program.
User exits (Function module exits) are exits developed by SAP.
Code for the function module is written by ABAP Developer .
Developer does not write code directly in functional module , but in the INCLUDE that is implemented in the functional module.