Presumably all retailers purchase products from vendors.
Rattail offers tools to assist with the various workflows involved. It splits the life cycle into 3 “phases” - Ordering, Receiving, Accounting.
Some purchases may not explicitly go through every phase. In particular it’s possible for the Receiving phase to include everything Accounting does, and in many cases Ordering is skipped (wrt the system), with the PO first “entering the system” in the Receiving phase.