I'd say that if you are using a mobile rig and antenna, you are mobile
whether or not you are in motion.  Portable operation would be from your
vacation home or from a hotel room.

If you connect your mobile rig to the triband beam on a tower, you can
call it what you like.

How about the hams who operate HF with a backpack rig and whip antenna?
I'd call it mobile even if the operator was sitting on a park bench.

Mark