Pre-cruise, your account should show how much you have in either/both categories. If you don't find any listed on your reservation, but believe you should, ask NCL (or your TA) to provide you with an "Amenities Invoice." Warning*** Be sure it you read that carefully, as the OBC and "Non-Refundable" often print on a separate line!
While onboard, if you find your account confusing, you can visit Guest Services for further explanation.
Generally, Non-Refundable Credit will be used for purchases in Gift Shops, Excursions (booked onboard) and Food/Beverage purchases that were not included in booked packages.