What is a purchase order (PO)?
A purchase order is a customer-issued reference approving work or goods before invoicing.
A PO is usually issued by the customer before work starts or before billing.
Some businesses will not pay an invoice unless the PO number is included.
If a client provides a PO, add it to the invoice to reduce payment delays.
Guidance only. PO usage depends on customer procurement rules and contract terms.
Legal review pending
Last reviewed: 2026-03-05