A Purchase Order (PO) number and an Invoice number are two distinct pieces of information used in the business world. A PO number is a unique identifier assigned to a purchase order by the buyer. It is typically used to track the progress of the order and to ensure that the seller is providing the correct goods or services. An Invoice number, on the other hand, is a unique identifier assigned to an invoice by the seller. It is used to track the progress of payment for the goods or services and to ensure that the buyer is paying the correct amount. While both numbers are used to track the progress of an order, they are distinct pieces of information and serve different purposes.