The crew appreciation that is sometimes included in HIA is the $16 per day per person that is automatically added to your cabin account for each day of the cruise. Yes, this crew appreciation is split between all crew members, including those behind the scenes.
When the fine print mentions the other gratuities, they are referring to any other services outside of HIA that you might use - for example if you were to order an upgraded meal item in one of the dining rooms (like a lobster dinner), you would be charged an additional 18% on the upcharge. Or if you went to a specialty dining restaurant outside of HIA, you would pay the fee for that dining option plus 18% service charge. Does that make more sense?