I mostly hear import guys use those terms. I guess it refers to level of upgrade
Stage 1 clutch is oem replacement
Stage 2 is better than stock with more clamping force
Stage 3 would be a race type clutch.
So the kc is a stage 2, but I have never called it that. Anyway, I have the kc clutch and like it. Good pedal effort, no issues. I also bought mine when they were cheap