A purchase order invoice is a document that is shared by the supplier when they’ve fulfilled the order for goods and services against a valid PO from the buyer. In informal terms, it could be considered a supplier’s formal bill that references the original PO associated with the completed transaction and lists the agreed-upon terms of fulfillment.

On the other hand, a non-PO invoice is a document with no reference to a PO associated with it. These invoices are usually a result of ad-hoc purchases like subscriptions or smaller routine, petty cash expenses, where a PO was considered unnecessary.

There could be cases where a formal PO process was bypassed, paving the way to dark purchasing and maverick spend. A purchase order tracking system will make such instances stand out like a sore thumb in a purchase order tracking software.