My tupp'orth:

'submissive' is an adjective, 'slave' is a noun.

A 'slave' (in the context we're talking about here) must be 'submissive' by thier very nature, albeit at one extreme of 'submissivness'. To the extent of being micromanaged and having no 'free-will' as has been mentioned before.

A 'sub' on the otherhand, while 'submissive' is not at that extreme end of the scale. They are someone to whom it means "being open and vulnerable" (thanks for the quote ) but still in control of and for themselves.

There, it took a while but I think I translated what was in my head into English *lol*