Purchase order or invoice?
A purchase order usually travels from buyer to supplier before delivery, recording the requested items, quantities and agreed terms. An invoice usually travels from supplier to buyer to request payment. If you are buying, you issue the PO; if you are selling, you issue the invoice. Payment or advance invoices can change the exact timing.
The Purchase Order Generator raises one with a number your supplier can quote back. Purchase Order Generator
Why the PO number ends up on the invoice
The supplier can quote your PO number on the invoice so the buyer can match the billed items to the order. Check the quantities, unit prices and delivery record too: the number alone does not prove the order was fulfilled.
For example, PO-2026-014 may request ten chairs at 80 each. If the invoice lists twelve, the buyer can compare the PO and delivery note before approving the extra two. Some buyers require a PO reference on every supplier invoice; ask for their process before billing.
Questions people ask next
Is a purchase order legally binding?
Whether a PO is legally binding depends on acceptance, standard terms, the parties' conduct and applicable law. Check the quantities, prices, delivery terms and cancellation terms before sending it.
Do small businesses need purchase orders?
Not for everything. They earn their keep once you are ordering from the same suppliers repeatedly, or when the person ordering is not the person paying.