I don't think it would be derogatory to call Adam Pascal an understudy, but it is inaccurate. An understudy is a member of the company who covers a role and is ready to go on at any performance, at a moment's notice. Unless I'm mistaken, that is not what Pascal is doing. He's been brought into the show for two pre-scheduled appearances. But if Andy Karl calls out on a random Thursday night, I doubt Adam drops what he's doing to come fill in.
My guess, as I stated in a previous thread, is that they're grooming Adam to take over the role when Andy eventually leaves, and that he's been willing to get his feet wet and learn the staging this way.