Record is like your join in a saved search when you go down to like "XXX Fields...." and then choose one. If you only need to get subsidiary from the Purchase Order, then you can leave the Record field blank.
If, for example, you needed the subsidiary that's on the Vendor record and NOT the Purchase Order (which is normally the same as on the purchase order, but imagine it's different lol), then you could put "Vendor" in the Record field, and it will look at the Vendor's subsidiary vs the Transaction (purchase order) subsidiary