Are you in some industry specific solution system? It appears that this is filled in when you create a PO with reference to a bid or RFQ invitation document. From a functional perspective, may be you invite vendors to bid for your order and you create a record for that invitation and then when you select a vendor based on the bid, you will create a PO for that vendor and you want to refer to your original bid/RFQ number.
Check with your functional counterpart as to where they create bids and how they create POs with reference to a bid.
Bluesky
