And, they collect this only if the upgrade clears (and at time of reticketing). So for example if you waitlist for an upgrade initiatially and then it clears a week before departure, then they charage it for you at the week point.

To the OP: I know you don't want to be told to "just do a search" but next time it may be worth your while to just put a little bit of effort in and search for something as simple as "luxury tax". I know for certain that there are a few posts on this very recently.