I think tire size would be a decent 'rule of thumb', but of course they're are exceptions. Take for instance the Kubota
M59 (soon to be
M62), it's technically a CUT, but it has more lift capacity than many utility size tractors that are larger framed, like the
M7040, and even on par with the M8540/9540. The
M59 has the same tire size as most CUT's. Of course, you could also classify the
M59 as a TLB, but you can classify any CUT or UT as a TLB as well. Pretty tough to come up with hard and fast designations.
I think it's virtually impossible to group what is a CUT, vs. Utility, similar to implements, how you classify something as light duty, medium duty, heavy duty, or super duper extreme duty. Since there is no standard, manufacturers can call them whatever they want.
Overall, I think the tire size is the best rule of thumb so far.