Receiving documents are different from DSD receiving documents.
Receiving documents typically come from purchase orders. It's not required to have created a purchase order first, but typically this is where they originate.
Purchase orders are created and given to the vendor, then when the order gets to your store, the purchase order becomes a receiving document that you use to check in the order and make sure quantities and prices are accurate. Once you complete a receiving document it typically becomes a billing document that the office uses to pay the vendor.