Difference between User Exit & a Functional Module

Question

Dear all,
Please help me in understanding ,what's an User Exit & a Functional Module in
standard SAP.How do we define both of them.
1.When  can/cannot I use an user exit?How to check for which T codes in PP
Module an User Exit can be used or present.
2.Can I use only a Functional Modules for Z developments or also an user Exit for
Z developments?
Please explain it in a simple way.
It will be a great help for me,Expecting for your help.
Regards
Mangal

Answer 1

user exit is a customised code used to extend or change normal SAP functionality.FM is a piece of reusable code uesed as a part of modularisation technique in SAP so that you can use this code again & again.
1. you create FM through se37. First create function group which in return will contain your FM. you creates user exits through SMOD and CMOD. In case of some standard programs there are special includes where you can add u r own code to extend standard functionality. ( in pro sapmv45a include sapmvfzz used).
to find an exit go to se80 or se84 give the package name and find exit corresponding to it ( ex for sales exits give package VA then search for exit )
for second point i didnt get get what u want to know?
for any z development u dont need at all to use exits u can do it thru SE38.USER-EXIT is add on functionality to customise u r standard transaction.
i hope this will clear u r doubt.reward points if helpful.
Rushikesh