hmm….. like the $PT stands for point, $PR stands for primitive, which stands for the group?

There isn't one, afaik. Also, even if there were something like $GROUP, a point can be in multiple groups at once, so that content of that variable would be more than one word. You'd probably also need $PTGROUP and $PRGROUP, though the point SOP would probably just supply $PTGROUP.

thanks for your help Antoine Durr