Unique text or a unique image on an invoice that INVOICES uses to identify which invoice definition to associate it with. Invoices with the same identifiers are from the same supplier.
An example of an identifier is a bank account number. If the invoices from a supplier always contain the same unique bank account number, this could be used to identify the invoice definition.
Another example is a supplier logo. If the invoices from a supplier always include the same unique logo, this can be used to identify the invoice definition.