In some countries invoice number is strictly defined by laws. I.e., invoice number is the first number, followed by some other numbers. For instance, invoice #34 could be 34-1-2, where 34 is invoice number, 1 is sales person identification, and 2 defines location.

It would be nice, if we can't define sales person and location IDs, if it could at least be possible to define suffix for invoices.