Purchase Order
Purchase Order Definition
A purchase order, or PO, is a buyer-issued document that records what the buyer is approving for purchase, including the seller, items or services, quantities, prices, delivery details, payment terms, and PO number. It usually appears after an internal purchase request is approved and before the seller sends an invoice. Once the seller accepts the order, the PO becomes the shared order record both sides use to fulfill, invoice, match, and pay the transaction.
For sellers, the PO matters because many customer accounts payable teams will not approve an invoice unless it references the right PO, matches the approved amount, and follows the buyer's portal rules. Buyers use POs to approve purchases before cash leaves the business. Sellers use customer POs to create invoices that AP can approve without manual cleanup.
Also referred to as: PO, purchase order number, customer PO, PO-backed invoice.
How purchase orders work
A clean purchase order process connects the request, approval, seller fulfillment, invoice, and payment. A team identifies a need for goods or services, the buyer submits an internal purchase requisition for approval, procurement or finance issues a PO to the seller, the seller accepts the PO and delivers, then the seller submits an invoice that references the PO number.
Accounts payable then matches the invoice to the PO and releases payment when the details agree. In everyday finance work, an accepted PO is treated as the agreed record of what was ordered, even when the legal effect depends on the contract terms and governing law.
Purchase order vs. invoice and related documents
A purchase order is one document in a larger buying and billing flow. A quote is the seller's proposed pricing and scope before the buyer commits. A purchase requisition is the buyer's internal approval request. A purchase order is the buyer's authorization to buy. A sales order is the seller's confirmation that it accepted the order. An invoice is the seller's later request for payment.
The simplest difference is timing. A PO says the buyer approved the purchase before fulfillment. An invoice says the seller delivered and is requesting payment after fulfillment. That distinction matters because AP teams often refuse to approve an invoice that does not match the PO they already approved.
What a purchase order includes
A useful PO removes ambiguity before the invoice arrives. Common fields include the PO number, issue date, buyer legal entity, billing address, AP contact, seller details, remit-to information, ship-to or service location, line-item descriptions, quantities, unit prices, taxes, freight, fees, delivery date or service period, payment terms, invoice instructions, terms and conditions, and approval evidence.
Invoice instructions deserve special attention. A correct invoice can still fail if it misses the customer portal field, required attachment, legal entity name, or PO format the customer requires. Sellers should store the PO number, balance, expiration date, portal, and invoice rules where billing and AR can find them before the invoice goes out.
Why POs matter for invoice matching and payment
PO matching is the AP control that checks whether the invoice agrees with the purchase order and, when relevant, the receiving record. A two-way match compares the PO and invoice for supplier, pricing, quantity, terms, and approved amount. A three-way match compares the PO, invoice, and receipt or proof of delivery so the buyer knows it received what the seller billed.
For sellers, the cash-flow issue is direct. If the PO number is missing, wrong, closed, or tied to the wrong entity, an approved invoice can sit in exception handling while payment terms keep aging. That is why PO work belongs in accounts receivable automation as well as procurement: the goal is to make the invoice payable the first time it reaches the customer's AP workflow.
Common purchase order errors
Most PO delays come from small mismatches that force AP or AR into manual exception handling. Missing PO numbers prevent AP from tying an invoice to approved spend. Wrong PO numbers route invoices to the wrong buyer record. Invoices above the remaining PO balance require a PO increase or change order before payment can move forward.
Other common issues include line items that do not match the PO, wrong legal entity details, tax or freight charges outside the approved PO, expired or closed POs, and missing portal attachments such as the PO, statement of work, delivery proof, or acceptance record. Ari handles this seller-side work by reading customer POs, matching them against invoices and contracts, submitting through AP portals, and routing exceptions in Slack.
When you'd use this
- Approving a purchase that needs traceability before cash leaves the business.
- Explaining why a customer requires a PO number before paying an invoice.
- Diagnosing rejected invoices, portal failures, or PO-backed collections delays.
- Designing controls for PO matching, AP approvals, or customer portal submissions.
Related Guides
See how Ari reads customer POs, matches invoices, and submits through customer AP portals.
Automate invoice follow-up, collections, and AR exception handling.
Net 30, Net 60, discounts, and other terms that determine when invoices are due.
Purchase Order FAQ
What is a purchase order?
A purchase order is a buyer-issued document that approves a specific purchase from a seller. It lists the seller, items or services, quantities, prices, terms, delivery details, and PO number.
Who creates a purchase order?
The buyer creates the purchase order. In larger companies, an employee submits a requisition, procurement creates or reviews the PO, and finance or a budget owner approves it before it goes to the seller.
Is a purchase order legally binding?
A purchase order can become legally binding when the seller accepts it, depending on the order language, governing law, and related contract terms. Finance teams usually treat an accepted PO as the agreed record of what was authorized.
Is a purchase order the same as an invoice?
No. A purchase order is created by the buyer before fulfillment to authorize the purchase. An invoice is created by the seller after delivery to request payment.
What is a PO number?
A PO number is the unique tracking number the buyer assigns to a purchase order. Sellers include it on invoices, packing slips, and portal submissions so the buyer can match the invoice to approved spend.
Can an invoice be paid without a purchase order?
Yes, if the buyer's policy allows non-PO invoices. Many companies pay utilities, card purchases, small expenses, or contract-based recurring charges without a PO, but enterprise customers often require valid PO numbers for vendor invoices.