what is the function of the body module sensor?

I believe you are referring to the purpose of the Body Control Module that is found in some GM vehicles. -- A body control module is a computer component in an automobile that checks, regulates and operates electronic devices throughout the car. When electric devices first began being used in cars and trucks, each device was governed by a separate electric component. There was a module for the air-conditioning, a module for the interior lighting, a module for the door locks, and so forth. A body control module gathers all these separate modules under one system so that they function together instead of separately, a format easier to both manufacture and troubleshoot.

ecm morphed to bcm

You'd rather not know. Best if you NEVER had to deal with this at all.