If you mark an item as “do not bill” in the customer billing preparation screen the bill quantity and $ are cleared and the item is marked as “do not bill” which means it will come up next time you run billing. If on the other hand you change the quantity to zero it will now marked at “fully billed” with zero quantity and $ which means you do not want to bill it now or later. You can also modify the quantity to bill and then mark the item as “fully billed” to avoid it being picked up again in a future invoice