here. Several rumours, so I thought I would document what I found.
When we were led on to the ship to Haven the butler did say there is priority elevator for Haven, but he had a laminated card saying Haven on one of the lifts holding it open, and we assumed he simply meant for boarding. No further explanation was given.
The forward/starboard (blue) elevators can get to the Haven (deck 16/17) and anyone can go to those decks. You do not need a card in the elevator. However, once on those decks, all the doors (even to the bar) need a card (and the readers are not good, so it can take a while).
The lifts also have a card slot outside by one set of call buttons, but I can confirm that really does seem to do nothing with guest cards. I tried and I saw others try. Also pressing combinations of door close and a floor do not help.
However, the card slot inside the elevator does do something with Haven guest cards!
This is how it seems to work - you insert the card and all floor selections and cleared and the display changes to say "PRIORITY SEVICE". Using card again cancels this mode.
You can then select a floor, the lift then goes to that floor, skipping all others, and goes back to normal mode. Simple.
No instruction is given on lift etiquette, so it gets complicated. Indeed, a simple "do not enter an elevator that is not going your way" would be nice...
What do you do if lift has people in it and they have floors selected. Using priority takes over the lift, and clears all their floors. Not polite. Worse, you could probably take over a down going lift and send it up to your floor, even more impolite!
So, what if there is only one person, say you are going from floor 5 and want 17, but someone in the lift has selected 15. Well, seems polite to help them, use priority and re-select 15 as well as 17.
There is a catch!
If you do that you go to 15 and it goes back to normal mode, and clears the floors again, including your 17. You could now find yourself on a down going lift. If you are quick you can priority and 17 again. If someone coming on to the lift picks 16 you have the same game. If they try lower floors they probably won't register, so being impolite again.
Basically, it works well if you (and your party) are the only ones that get in to the lift in the first place, and so often works well if you are getting on at 17 and going down. Almost any other combination is at best impolite, and at worst means you end up not getting to your floor.
P.S. in spite of previous experience of someone getting on an up going lift at 16 and selecting lower floors, I have now seen many times that trying to select a lower floor on an upping lift does not register the selection (light on then off). Maybe the 16 to 17 is an exception simply because it will be down going in one floor guaranteed, with 17 being top floor. Maybe the lifts are each working in different modes, not clear.
One of the lifts has the mechanism to read the card missing/detached, leaving a hole in to which you are welcome to put your card if you never want to see it again. I am sure they will fix it soon.
I would suggest a small card or something that says how to use the priority service, and some tips own etiquette...