Original inhabitants of Lambda can take "jobs", and gain "levels" in them (100 levels per job). 3 jobs makes a competent adventurer, 5 jobs makes an elite knight.
Monsters have a knock-off of that system where they can get "ranks", each rank about as good as a completed job. For monsters, increasing rank changes their race name (to something like "goblin archer" or "ultimate god-slaying bone horror"), and their appearance.
Some races have access to both at the same time, allowing them to progress about twice as fast. Because they're descendants of monsters and more wholesome beings (like Vida). 
This is not synonymous with "Vida's races", as Dark Elves (descendants of elves and Vida) are a Vida's race but they aren't descendants of monsters, and so they level up like ordinary humans and elves (but are a little stronger from the start). There should be other Vida's races like that too, but I don't remember which ones.