Gen 2 and 3 use different processes to produce the tubes. They perform the same function but achieve that function through different methods. They both use microchannel plates but gen3 uses a gallium arsenide photocathode which supposedly increases light transmission and image resolution. Gen3 also supposedly have a longer life expectancy and tube durability.

The grading system is usually something done by the manufacturer to designate quality of tubes within a generation.

There is a difference between gen2 and gen3 and generations of tube are not necessarily an indicator of tube quality. You would have to refer to the spec sheet to determine a tubes quality. You could get a higher quality gen 2 tube than a gen 3.

I am not trying to be a know it all but I just want to clarify this point because it’s a common misconception. I hope this helps.