APO Planning Cheak

Hello Gurus,

While carring out SNP Planing run it will check for the availability of the material,capacity available for planning, I had a query about how the system fetches the data from system and at what locations it will look for the check e.g. whethet it will cheak for Pur.Req.,open POS, storage locations or some other sequence? How can i find the where system searces for the data?

Thank You in Advance..

Amol Chavan