In this context, PO stands for “purchase order.” The difference between these two types of invoices therein lies the type of purchase that was made.
So, a PO invoice should include the purchase order number and details of what the customer is purchasing. It’s a fairly simple, direct invoice.
A non-PO invoice is used in the case of indirect purchases. It does not have a purchase order number and has not been pre-approved. This type of invoice is also known as an expense invoice.