My understanding is that sublimation is channeling unacceptable drives/emotions into socially acceptable actions (eg, want to kick a puppy but instead enroll in a karate club).
Reaction formation, on the other hand, is characterised by denying/avoiding the unacceptable drive/emotion and parading a directly opposite attitude (eg, want to kick a puppy and become an animal rights activist

).
Sublimation is mature; reaction formation is an immature defence mechanism (so, you cannot really say that reaction formation is subset of sublimation).
I am sure there is a better explanation, but I can't think of one right now.
I might be wrong though.