According to Wikipedia (http://en.wikipedia.org/wiki/Purchase_order):
A purchase order is a commercial document issued by a buyer to a seller, indicating types, quantities, and agreed prices for products or services the seller will provide to the buyer. Sending a purchase order to a supplier constitutes a legal offer to buy products or services.
Creation of a purchase order document can be done in two ways:
Open the Menu | Requisition-to-Invoice | Purchase Order window. There will be five tabs: Purchase Order, PO Line, Matching, Requisition Lines, and Order Tax. If the Show Advanced Tabs checkbox is in a deselected state in Tools | Preference, then this ...