Each document will has its own document running number. You may set your preferred format and manage the last running number to continue in SQL Account.
Scenario A
Sales Invoice INV0817/001 (Format: INVmmyy/001)
Go to Tools.
Select Maintain Document Number
Select Customer Invoice > Press Edit.
Make sure the Format is INV{@mmyy}/%.3d
(Optional) Change Frequency to Reset Monthly if you wish to reset the running number every month.
You may fill in the Next Number so SQL Account will use this as the next number to generate invoice.