Humans can be any class so the "legacy" unlock for them gives you +100 presence, for all the other races when you get a character of that race (any class) to 50 you can unlock them for use for any class they can't normally be. So, for instance, you could lvl a Sith (Race) to lvl 50, get the unlock, then you can play as a Sith (Race) Jedi Knight, or BH, or Agent, or w/e. The same goes for any of the races.