Folks in nearly every sanctioning body have found that it makes sense to form displacement windows for vehicles competing in certain types of motorsports.
It helps make sure that the competition is fair to all those competing directly against the other competitors in the same class of vehicle.
NHRA and NASCAR found that it was necessary in order to equalize the competition. What they found was that in certain cases, smaller engine displacements had a distinct and unfair advantage over larger displacement vehicles running the same class. The same thing could, and does apply to any LSR class, thus the need for displacement windows.
I agree with the term unlimited and/or universal, but context is important too, and in this case the term unlimited means what it imply s, any thing goes as long as it's over 3000 ccs.
Rouse