It helped me to think of them as a "Magic Token" that the Finance Department issues so they know that the invoice is genuine. It means that when the invoice ar

It helped me to think of them as a "Magic Token" that the Finance Department issues so they know that the invoice is genuine. It means that when the invoice arrives they can look up the appropriate file and check that the details before issuing the payment. A busy Finance Department needs some way to cross-reference incoming invoices against previously issued permissions.