It's not like there's an industry standard, these are "bins" that individual companies define.
But as an example, this is how Beverage Elements bins their kegs (when they have any):
Class 1 Kegs: Premium dual rubber handles, fewest scratches and dents
Class 2 Kegs: Single plastic handles, more scratches and dents
Class 3 Kegs: Cut single handle, most scratches and dents
Cheers!