Suppose I have a script which is monitoring a directory
whenever a file drops in that directory,it sends alert

say I want to write a return code for the above script
which on successful execution of script gives a return value

Based on return code , I want to do initiate some jobs in other systems

how to do that?

This is a decision that is for you to take. The convention is that you return 0 on successful and -1 if not. If you have multiple exit statii (--cringe--), you can decide what exit status will stand for what.