To see all the numbers that Invoices is capable of printing on an Invoice, print out your invoice using the "All spec codes 9.0A 2011" specification. If it's not included with you data, you can find it as a Template. Just go to the specification editor, Click "Open Template" and save it with your data. Then print out the invoice and see whether the number you need is available. If it is, then edit your form specification to include the number on your production invoice layout.