I was hoping someone could clarify a rule in their limitations in their Rite of War.

It says they can not field and unit with the Immobile, Heavy or Slow and Purposeful Special Rules.

The only place I've noted a special rule of Heavy is in regards to weapons, and it clearly does not refer to that. I'm assuming it has something to do with movement, but in going thru the Legion Unit Lists, I'm not seeing anything anything with that rule, either. Does it refer to, say, Super Heavy Tanks?

Thank you very much, I appreciate the help, in advance. I'm guessing it's going to be something stupid I overlooked about 50 times while looking through stuff.