I agree with schmism's point that horsepower is a very crude measuring stick. I think that's especially true down in the riding lawnmower size machines.
Size, tire size, and weight are also important, but those should roughly correspond to horsepower. Maybe we should define the terms based on what the machine can do (to expand on MikePA's ideas). Can it handle a 5' brush hog with ease? Can it square-bale with ease? etc.
The "with ease" parts are the gray lines between the definitions.
When I bought my tractor I looked at what it could do for me and I didn't even know the terms CUT or SCUT.
This reminds me of the terms microcomputer, minicomputer, and mainframe or PDA, laptop, notebook, desktop or jump drive, pin drive, USB drive, thumb drive, memory stick, etc. Sheesh!