Invoice admin

Invoice number, PO number and payment reference: which is which?

Your invoice number identifies the bill you issued. A purchase order number identifies the customer's order. A payment reference helps connect a transfer to the payer or invoice. They can refer to the same job while remaining different identifiers; label each one instead of treating them as interchangeable.

By PayerPing · Published 11 November 2026

Identify the record behind the number

When accounts asks for “the reference”, ask which record they are trying to find. They might need your invoice number to locate the bill, their PO number to find an internal approval, or the text used on a transfer to help investigate payment. Supplying a different number can leave the original problem unresolved.

GOV.UK's invoice guidance includes a unique invoice identification number. The Small Business Commissioner separately advises requesting a PO early when the customer needs one. That does not mean every customer uses a purchase order, or that a PO replaces your invoice number.

A payment reference may repeat the invoice number, but check the actual transfer record. A payer might instead use a supplier account code or a shortened name. A useful reference is evidence to compare with other details; it is not, on its own, confirmation that a particular invoice is paid.

A fictional job with three different references

In this fictional example, Cedar Studio supplied a brochure layout to Marsh Events for £640. The owner has checked the invoice, the customer's order and the received transfer separately.

Fictional reference crosswalk for one £640 job
Label	Example	Where checked	Question it answers
Invoice number	CED-1042	Cedar's issued invoice	Which bill did Cedar issue?
Customer PO	ME-PO-773	Marsh's confirmed order	Which customer order is this for?
Payment reference	CED1042	Received transfer dated 10 November	Which payment might settle this bill?

The missing hyphen in CED1042 is a reason to compare the amount, date and payer, rather than reject the possible match automatically. Equally, a transfer for £640 should not be assigned solely because one open invoice happens to have that amount.

Keep a short, labelled crosswalk

Copy this into your normal invoice records. Keep the customer's reference exactly as confirmed, including meaningful prefixes or leading zeros. Record an unknown reference as unknown instead of manufacturing a value to fill the field.

Invoice number: [your issued invoice number]
Customer / job: [customer and identifiable work]
Customer PO: [confirmed number / not required / awaiting confirmation]
PO confirmed by and date: [known contact, date]
Requested payment reference: [reference shown in your payment instructions]
Actual received reference: [exact text / receipt not yet identified]
Receipt checked by and date: [owner, date]
Confirmed invoice match: [invoice and amount / unresolved]
Next check: [specific question and review date]

Follow up on the missing link

If the customer needs a PO that you do not have, use the missing purchase order workflow. If money appears to have arrived, use the payment check before updating the invoice or sending another chase.

A narrow request works best: “For invoice CED-1042, do you need the purchase order ME-PO-773, or the reference used on the 10 November payment?” Once the owner confirms the match in their normal records, they can update the reminder record. PayerPing does not inspect bank transfers or make this matching decision for them.

Sources and further reading

All invoice reminder guides