Why does every document have both 'Sent' and 'Received'? What's the difference between an original and a copy?

Each document exists as one copy with each party: an original is the document you send out, which appears in the 'Sent' page of the list. A copy is the document the other party sent to you, which the system automatically generates on your side and appears in the 'Received' page. You cannot create a copy manually; it only appears when the other party sends it. These are two independent documents: when you modify your copy, it will never directly modify the other party's copy. The other party only sees the version you sent out last. Which party is the buyer and which is the seller is determined by the document type: PO and inquiry are opened by the buyer, while PI and quotation are opened by the seller. For example, 'Received (PO)' is a PO the customer opened for you, making you the seller. The sidebar link will remember whether you last viewed the Sent or Received page.