Basically anything that you purchase onboard with your sail card. That would be pictures, shore excursions, gift shop purchases, and drinks. They may say you can't use it toward gratuities but I highly doubt anyone is sitting there looking at each account when they run the final bills.
On most cruise lines, the onboard credit appears as a credit on your onboard expense statement, and as such, is an amount subtracted from whatever you charge onboard. If the gratuities are on the onboard account, rather than paid in cash, it's all part of the accounting.
To be sure, I suggest calling your travel agent, or calling NCL directly to ask about their procedure.
We had a small on board credit last year and put it on our shipboard account. It was then deducted from our total which included tips, shore excursions, drinks, etc.
I have also seen coupon booklets which are specifically geared to certain items - eg. 10% off picture or free frame with picture purchase or discount on spa treatment.
Twice I have had on-board credits from NCL. They have always stated that they cannot be used toward gratuities, so I'm sure yours will state that if that is the case. Fortunately, or perhaps unfortunately, we have always managed to spend enough on board through photos, extra price dining or a few drinks that it really didn't matter. It is a nice feeling to have a drink or make a little purchase on board and know that the on-board credit is paying for it.
Enjoy your cruise