What is required when creating a PO tied to BPA?

Summary:

When a user creates a stand alone PO (no req backing it), what field are required in order to tie it to the BPA? If the BPA does not have a supplier item entered in, how can I still tie it to the PO?

Example: BPA is for a fixed priced service. User creates a stand alone PO for the supplier and adds a fixed priced line, but cannot select the BPA. What is missing?

Also, can we restrict users to only be able to use the BPA if the correct purchasing category is selected?

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