What are Optional Vouchers in Tally ?

Optional Voucher

This is another non-accounting voucher. However, it differs from the Memo voucher in that it is not a separate voucher type. You mark an existing voucher, e.g., a payment voucher or a receipt voucher, as Optional. Press [Ctrl]+]L] or click on the button. This button toggles with Regular. By making it Optional the voucher does not get posted anywhere but remains in the have the Optional Register. You can make a voucher type default to Optional.

This is done during creation of new voucher type or alteration of an existing voucher type.

Gateway of Tally>Account Info>Voucher Types.