If memory serves, previous poster is right. This is just a recommended amount. RCI does NOT auto charge this to your sea pass, although you can ask to have it charged to your account by visiting guest services, after which they give you a chit to put into your envelope. I believe that when we did this, we were told that if you charge to your sea pass, you MUST do the recommended amount (I don't recall if you could increase it). If you want to tip lesss than the recommended amount, then you must tip cash.
I kind of like the fact that some lines auto-charge this to your account so you don't have to really think about it. It still floors me that so many people skip dinner on the last night (presumably) so they don't have to tip.
I don't believe this is true, but on Carnival DReam, our waiter told us that he did not get the auto-tips, and that if we were pleased with his service, we should tip him cash. We checked with guest services and they said that was false and that he received his portion of the $10 per day. What bothered me with him, is that prior to getting into the tip discussion, he asked how much our house was worth ... that he wanted to move to Canada but he was not rich like us .... THAT really turned me off (together with his lie about not getting the auto-tips). Nobody wants to feel guilted on a holiday.