Purchase order vs invoice number
Purchase-order number vs invoice number: two different references
Direct answer. A purchase-order number is the buyer’s reference on the document that authorizes a purchase. An invoice number is the seller’s reference on the document that asks for payment. They are issued by different parties at different moments: the PO number identifies the buyer’s authorization to buy, and the invoice number identifies the seller’s request for payment. General information only; not legal, credit, or accounting advice.
B2B PO Guard — Require PO (Shopify App Store · Write a review) validates PO numbers by format, prefix, regex, length, and uniqueness, and previews any PO rule live against a sample number before you publish. This page is the PO-versus-invoice distinction — not the require-PO job and not the regex catalogue. Requiring the checkout field is why a B2B checkout can skip a PO number unless a rule requires it.
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. Test prefix, length, characters, regex and duplicates on sample PO numbers.
Who issues which number
| Purchase-order number | Invoice number | |
|---|---|---|
| Issued by | The buyer (their purchasing system) | The seller (their billing system) |
| What it identifies | The buyer’s authorization to buy | The seller’s request for payment |
| Typical moment | Before or at order, so the seller can put it on the order | When the seller bills, often after fulfilment |
| Shopify B2B checkout field | The PO number field on a company checkout | Not that field |
Buyers set their own PO formats. There is no universal pattern. Example shapes and regexes live on purchase-order number format examples. An invoice series is a different numbering scheme; do not paste invoice samples into a PO format rule and treat a match as a valid purchase order.
What a checkout PO rule can and cannot do
A Cart and Checkout Validation function can read cart.poNumber and reject a company checkout when the field is empty or fails the pattern you publish. That is a PO-number rule. It does not issue invoices, does not match the string to a buyer’s purchasing system, and does not prove the order was approved or paid.
Format, prefix, regex, and length are the slice on validate B2B PO format, prefix, regex, and length. Uniqueness (the same PO used twice) is prevent duplicate PO numbers. Preview the live rule against samples before you publish, from the B2B PO Guard — Require PO listing.
What this page is not
- Not a tax, VAT, or accounting standard, and not a chart of accounts.
- Not a check that a PO number is real, approved, funded, or unique in the buyer’s system.
- Not the require-PO, uniqueness, or regex-example siblings.
- Not a postal PO Box block. That job is block PO Box on billing.
Related pages
- PO number format tester
- Purchase order number format examples
- require a PO number
- Require a B2B purchase-order number
- Validate B2B PO format, prefix, regex, and length
- Stop reused PO numbers
- Preview a PO rule before publish
- Shopify B2B payment terms, including Net 30
- B2B PO Guard — Require PO — live App Store listing
- Shopify apps hub
- Tools
General information only. Not legal, credit, or accounting advice. Confirm the identifier your buyer and your billing system actually use before you treat a string as a PO or an invoice.