Rick, I like the question. It is a useful one too, in trying to figure out what a person's true competency is. We know that learning this work is not a merely academic enterprise: you don't just read a book. The ability to climb a tree, climb a tree and work in a tree, climb in a tree and rig it out, etc. varies greatly with indiviudal person's, sometimes regardless of their time spent doing "tree work". I hire people as tree climbers, but don't impose any classification on them. To be a climber implies a certain kind of personality that not all groundpeople have. You've got to show nerve and guts and attention to detail, in my view, as a precondition for being asked to climb. These attributes must be there in order to move to the climber rank. I've been thinking in my mind for years of the following classification but don't impose any formal system on them yet. Try to keep things fluid so people feel like they can move up the ranks by their own inner development.
Tree Climber I: intro level, ddrt, some pruning; semi competent must be supervised
Tree Climber II: ddrt, srt, pruning, small removals; semi competent must be supervised
Tree Climber III: ddrt, srt, pruning and all removals, fully competent suitable for Crew Leader