Purchase Order when the buyer agrees by Proforma Invoice prices and terms, and then to send the book (Purchase Order), the vendor to provide a receipt and order items by price and such terms.
Purchase Order when the buyer agreed to the price and terms of the Proforma Invoice will send a letter (Purchase Order) to the seller to provide a response and orders by price and Conditions.
Purchase Order when the buyer agreed upon price and terms in the Proforma Invoice then send the book (Purchase Order) to the seller so as response and order by price and such conditions.