User Exits in SAP
SAP User Exit is used to make some changes when SAP standard functionality is not able to fulfill the requirements.
User exits are empty subroutines where a developer can write custom code for the business requirement.
This is implemented in the form of subroutines and is also known as FORM EXITs. Generally, These are collected in includes programs and attached with the SAP standard program.
We can find User Exits in SAP Transaction code SMOD.