Prepaid gratuities cover what you would normally tip onboard for your stateroom attendant, waiter, assistant waiter, and headwaiter. If you receive service somewhere else (bar, spa, room service, etc) the tips for those are not included with the prepaid gratuities.

If you buy a drink at a bar, they will automatically include 15% gratuity in the bill, and have a line for additional tip. I believe the spa automatically does 10% gratuity now (not certain of the exact percent). Room service you should tip in cash on delivery.