A Purchase Order number (commonly referred to as a P.O. or PO number) is a unique, identifying number assigned to a Purchase Order so buyers, suppliers, and vendors can keep track of orders.
Small businesses may develop a basic numbering system, such as assigning PO numbers sequentially (e.g., 000001, 000002, etc.) and manually keeping track of them. Larger businesses may use accounting software that automatically assigns and tracks PO numbers.