Manager automatically applies a serial number to sales invoices. The program searches for the highest existing sales invoice number and increases it by 1. You can override this default serial number if you want.
If you want to customize your serial numbers, such as starting with a specific number or including a year number, simply override your first invoice. For example, enter 2015001 on your first invoice. The next will be 2015002. You cannot, however, enter alphanumeric numbers like AB001 and get AB002, because Manager treats the serial numbers only as numbers. If you need such alphanumeric serial numbers, you must enter them manually.
Purchase invoices are not currently editable. But the format includes a Reference field you can use for anything you like. This is not numbered automatically, though, because purchases invoices record amounts invoiced to you by a supplier. The reference you should record is the supplier’s number, not your number. There would be no point to including this in a column of the purchase order, because every invoice should be recorded in a separate transaction. That is why the reference number appears in the header.