What is a PO number

A PO number is the buyer’s reference for an order it has authorised

Direct answer. A PO (purchase order) number is the buyer’s reference for an order it has authorised. The buyer issues it, and a seller typically quotes it back on the invoice. In Shopify B2B checkout, the customer can enter one. Shopify Help says: “Customers will have the option to enter a purchase order number for the order.” US federal procurement uses its own definition, quoted below. That definition is not a universal one. This page is not accounting, tax, or legal advice.

B2B PO Guard — Require PO (Shopify App Store · Write a review): “Validate PO numbers, deposits, Net terms, order review, and D2C no-op behavior for Shopify native B2B checkout workflows.”

Related: PO Box Blocker — AddressFence — Free blocks PO Boxes and parcel lockers on the shipping address at checkout, plus a small set of ZIP rules. Street-number checks and billing-address rules are on a paid plan.

Free check first: PO number format tester runs in your browser and sends nothing. Paste a sample number and see whether a pattern you type would accept it.

Checklist before you treat a PO number as one kind of field

• The buyer issues the purchase order number. The seller’s invoice number is a different reference: Purchase order vs invoice number.
• FAR 2.101, quoted below, is the US federal-procurement definition. It is not a definition of every commercial PO number.
• On Shopify B2B checkout, Help says the customer has the option to enter one. Requiring or validating it is a separate rule: B2B checkout can skip a PO number unless a rule requires it.
• Formats are the buyer’s. Examples already collected are Purchase order number format examples. This page does not add format rules.

US federal procurement: FAR 2.101

FAR 2.101 defines terms for the Federal Acquisition Regulation. The definition tooltip for “Purchase order” on that page says: “Purchase order, when issued by the Government, means an offer by the Government to buy supplies or services, including construction and research and development, upon specified terms and conditions, using simplified acquisition procedures.” That is the US federal-procurement definition. It is not a universal definition of a commercial purchase-order number.

US federal invoices: FAR 32.905(b)(1)(iii)

FAR 32.905 is the Prompt Payment rule for government contracts. Paragraph (b)(1)(iii) says a proper invoice must include: “Contract number or other authorization for supplies delivered or services performed (including order number and line item number).” That sentence is about invoices under those government contracts. It is not a rule for every commercial invoice. How a seller’s invoice number differs from the buyer’s PO number is Purchase order vs invoice number.

Shopify B2B checkout and draft orders

Shopify Help on B2B checkout says: “Customers will have the option to enter a purchase order number for the order.” The same page says: “Customers can also view the purchase order number for the order, if one was entered at checkout.”

Shopify Help on B2B draft orders says: “Optional: To add a purchase order number to your draft order, complete the following steps: In the PO number section, click the icon. Enter a purchase order number, and then click Done.”

Requiring or validating the number is one sentence here, and the rest is on B2B checkout can skip a PO number unless a rule requires it. The Shopify changelog dated April 1, 2026 says: “As of API version 2026-04, you can validate billing addresses and purchase order (PO) numbers in Cart and Checkout Validation Functions.” The same changelog says: “Input graph fields: billingAddress and poNumber are available on all Shopify Function input graphs. They return null outside of Cart and Checkout Validation.” And: “Checkout surfaces field-level errors for billing address and PO number when your function returns validations for these targets.” One target it names is $.cart.poNumber. Uniqueness, and format, prefix, regex, and length, are duplicate PO numbers and format, prefix, regex, and length. Net terms are Shopify B2B payment terms. How this app compares with another PO app is B2B PO Guard vs PO Validator B2B.

A PO number on a credit card

The card issuer or the payment processor defines any purchase-order field on a card. This page does not cite a card-network rule, because none is quoted here. On Shopify draft orders, a credit card is a payment method, not a PO field on the card. Help says: “If the customer uses a credit card as their payment method, then click Enter credit card, enter the credit card information, and then process the payment.” The PO number on that same page is the separate optional field quoted above.

What this page is not

• Not accounting, tax, or legal advice.
• Not a universal definition. FAR 2.101 is the US federal-procurement definition.
• Not a new list of number formats. That URL is Purchase order number format examples.
• Not the invoice-number page. That URL is Purchase order vs invoice number.

Related pages

• PO number format tester — paste a sample number in the browser
• Purchase order vs invoice number
• Purchase order number format examples
• require a PO number
• B2B checkout can skip a PO number unless a rule requires it
• Prevent duplicate PO numbers on Shopify B2B checkout
• Validate B2B PO format, prefix, regex, and length
• Shopify B2B payment terms
• B2B PO Guard vs PO Validator B2B
• FAR 2.101
• FAR 32.905
• B2B PO Guard — Require PO — live App Store listing
• Shopify apps hub
• Tools

Not accounting, tax, or legal advice. Confirm FAR 2.101 and FAR 32.905 on acquisition.gov, and the live Shopify Help pages, before you treat a PO number as required. The app validates numbers against rules you publish. It does not issue purchase orders.