a "sub" is neither a boy nor a slave in MY opinion.  They are seeking more just the sexual high of being submissive for a shorter period of time.  They will go from one Dominant to another, without necessarily any devotion, emotional connection or desire to submit long term.

a "sub" will tend to be a player.

Now there is nothing wrong with this if you are seeking noncommittal submission, it just doesn't have in MY opinion the long term piece that a "boy" or a "slave" might have.