I'm not following the discussion so far. I'll just answer op's question as simply as I can.
As you may know, being part of this community for a while, animation as an art has rules, principles and concepts.
Animators with none or little knowledge of these and lacking ability to apply them to their work are called beginners.
A bit above, animators with an extended knowledge of these, and capacities to apply them to their work are called intermediate.*
Animators with an extensive knowledge of the art, that show domination of the principles are the advanced ones. Those that bend, explore and push the boundaries of the art to the limit are the ones we should call masters or experts.
Now for the "low/mid/high" divisions. I believe they are just there because it's impossible to measure with precision someone's skill. Someone may show domination of a principle, but completely lack on another, or maybe someone is inconsistent in the quality of his work. So we apply these subjective divisions which diminish the chances of wrongly qualifying someone.
How do we rate someone? Animation is a visual art. No matter the tools or method, an animator is always chasing a visual concept. This is what makes it impossible to truly rate an animator, because you must not only take into consideration his dominium of the principles and how he applied them, but also what he wanted to achieve and if he achieved it. What I mean is that, when rating someone, your concern should be "Does it look good?". How good it looks defines your level. OFC "good" is also a subjective term subject to the animator's objectives.
*Intermediates can be easily confused with advanced because they tend to stick with something and master it. Falling into a comfort zone where they do what they know will look good either because they've seen others do it or because they've done it before themselves. Once they start innovating they start trascending into the next level. (I'd say around 97% of stickpage belongs here. Not taking beginners into the equation.)
TL;DR
Beginner: Low grasp of the animation's principles and difficulty to apply them.
Intermediate: Knowledge and decent domination of the principles. Usually fails to innovate and mimics stuff he knows will work.
Advanced: Extensive knowledge of the art. His dominium over the art allows him to apply things in creative ways.