IDR - Supplier Site selection logic when forcing Supplier via Email Subject

Our organization would like to enable IDR exclusively using the “IDR SUPPLIER DERIVATION IN EMAIL SUBJECT” feature made available in 24A (Documentation here: 24A - IDR Supplier Derivation). This means telling IDR the correct Supplier by prepending Supplier Number, e.g. (12345), to the IDR email subject line.

Our question is: how does the Supplier SITE get selected when we tell IDR who the supplier is?

What we are getting is not always the correct SITE and we would like to understand how this is working behind the scenes.

Looking for the logical tree such as:

• Attempt to match Supplier Site address to an address on the invoice image