A purchase order is a document sent from a buyer to a seller, with a request to order a product. The purchase order often has its number, description and quantity of the goods, unit prices and total price, name of issuer, time of delivery, standard terms and conditions, etc. It is effectively an offer to supplier. The purchase order will become a formal contract if supplier accepted it by written notice or by performance (such as deliver the goods to the buyer's premise).
Reference: CIPS study guide page 33
LO 1, AC 1.2