It's attempting to describe how the tube will behave with it's natural current draw.
Like Groove Tubes, the Early are hotter natural current draw and claim to distort/clip/compress
at a lower master volume setting. Average is a midrange natural current draw and will bias normally.
These are claimed to be a good mix between early and late for most playing styles. The late distortion is
claimed to have a cooler natural current draw and may take a good turn on the bias pot to get into an
average of 60-75% MPD. Most high gain metal players want these as they have higher headroom and tend
not to compress, even at relatively high volume.
Basically, it's how vendors sell hot, medium and cold tubes. This works well for Mesa type guys who cannot adjust their
bias. Actually knowing a tubes natural current draw and even transconductance will help you know how quickly
it will bias. The higher plate current the less you will turn the bias pot and colder you will turn it more to get to the same
MPD. Some people look for the same plate current and transconductance when replacing a know good sounding set of tubes.
The effect of each class and how you bias them will be the final judgement for your ears.
GT has used this guide. It represents a decent explanation after biasing.
