1 - If the Invoice Numbering method is set to Master - The invoice number is created when the itemized invoice is printed. So, because this invoice is printed last (after the credit card slips), the invoice number may not be generated at the time that the system records the payment, but will appear in the customer record. Also, in this case if you exit the invoice without printing, then the invoice number will not be assigned. 

2 -  If the Invoice Numbering method is set to Auto - The invoice number is created when exiting the invoice. This also could cause a similar scenario as above, except that the invoice number will be assigned even if you exit without printing. 

3 -  If the Invoice Numbering method is set to Before - The invoice number is created when the invoice is created. This will allow for the invoice to be numbered immediately when you create an invoice but may prevent you from deleting an invalid invoice. In which case the invoice can only be voided, resulting in the invoice number being preserved and marked as a voided invoice number. The next invoice created, will be assigned the next available invoice number (incremented by 1). So if an A type invoice is created it will be assigned a number, e.g. A0004. If that invoice is voided the number will become N0004 (N is the designation for void). The next A type invoice created, would be assigned A0005 (or C0005 if it is a cash invoice).

4 -  If the Invoice Numbering method is set to Printer - This is an outdated setting that is not used anymore with Windows. In the DOS days of System Five, We used to have 1 number sequence for printer 1, and another sequence for printer 2, so you could have 2 printers with pre-printed, pre-numbered invoice numbers. Again this setting is not supported under Windows Operating Systems, as Windows no longer supports this setting.

5 -  If the Invoice Numbering method is set to None - The invoice number will need to be created Manually. This method will create all invoices with the number ASSIGN, and you will need to open each invoice and assign a number to them.