User exit is a program to pass control from Report Builder 
to a program you have written, which performs some 
function, and then returns controlto Report Builder.

We can write UE in before report trigger to populate 
the value.

User Exits are used to pass control from report builder to 
outside program.

   Basically if we want to create any flex field reports. 
we need to get the structure and code combination and 
description of flexfileds at that time we use userexits.

srw.flex_sql,flexid_val,srw.init  etc..