Glossary · Invoices and billing
Invoice number
An invoice number is the unique reference that identifies one invoice, so supplier and customer can find, match and chase it without confusion.
Every invoice needs a unique number. In the EU and UK, VAT rules require sequential numbering with no duplicates; elsewhere, a unique and consistent scheme is still what accounts-payable teams expect.
The invoice number is what your customer’s AP team searches for when you chase. Put it in the subject line of every reminder and in the bank details as the payment reference you ask the customer to use. That last step alone makes cash application much faster.
Prefixes such as INV-2026- and separate series per entity are fine, as long as each series runs in sequence. Never reuse a number for a corrected invoice: issue a credit note and a new invoice instead.
Example
A reminder titled “INV-2026-0142 — €8,000 due 2 April” is matched by the client’s AP clerk in seconds; one titled “Quick follow-up” gets forwarded around the building.