What is the logic for defaulting the Source Type while adding items from Category in RSSP?

Summary: We want to understand how the Source Type is determined for Requisition when we add an item to cart from Shop by Category option?

Content (please ensure you mask any confidential information):

We understand that when we enter item using Enter Requisition Lines, we get option to choose the Source Type - Supplier or Internal. But while adding item to cart via Shop by Category option we see inconsistent behavior. For some items the source type gets locked to Internal and for some it is locked to Supplier and there is no option for us to change this.

Version (include the version you are using, if applicable):