When I started almost 20 years ago I seem to remember it actually being a Class 2 Certificate. Somewhere along the way it changed to an "ATC" Certificate. I might be misremembering but I think that's how it was.
In any event like others said, whatever it is now is pretty damn close to a Class 2, but possible a bit more strict.
I seem to remember at one point pilots started being able to take the handful of big name anti-depressants like Prozac, etc. but ATC still cannot. Something like that, maybe.