A PO number or Purchase Order number is a unique number given to a particular order placed before the seller, similar to the invoice number. It is a reference number that helps both the buyer and seller effectively store and keep track of their purchase orders. The PO number is usually written on the top of the purchase order and is used as a reference to track the transaction. It is a legally binding document between the parties involved in the transaction. The PO number is most often found in a specific location at the top of the sales invoice, below, or beside the invoice number. It should be labeled as the PO number to avoid confusion. The PO numbering system is fairly straightforward. The buyer can create a PO number, and many will simply start with 0001, then 0002, 0003, etc. . Some small business owners may add letters to their PO numbers as well to further differentiate between departments or types of stock. The PO number is useful for administrative purposes in a business organization and is beneficial for admin and finance tracking.