Question - Explain the concept of User Exits?
Answer -
User Exits are a type of system enhancement that was originally developed for Sales and Distribution (SD) component. The original purpose was to avoid Modification Adjustment by the user.
It is a modification, as it requires you to change objects in SAP namespace. It creates a special include in module pool. Naming convention of the user exit is: USEREXIT_.
After delivering, SAP never altes includes created in this manner. If new user exits have to be delivered in new release, they are put in new include program.