Emerald Power: The Omnitrix's signature color is green, which also happens to be the favorite color of both Azmuth and Ben.
Likewise, the original series depicted it as growing with its wearer (the first seen Ben 10,000's was bulkier than both those of Ben 10 and Ken 10), a detail which none of the later series incorporated. In the movie, the dial has a column of hardware attached when Azmuth removes it. In "Back With A Vengeance", it unhinges more measuredly but from the back. It unhinges from the front like a blob of slime, lacks the green/black circuitry, and the dial/faceplate is an isolated piece of hardware.
Though others can be unlocked later, it will default to another set of ten if left inactive for too long, requiring older forms to be re-unlocked.
Though by the last season of Alien Force this is no longer true, as Ben knows exactly how to remove the thing whenever he wants. This prevents villains from simply taking it from him.
This frequently causes him to take the wrong transformation, turn back to normal at the wrong time, or merely make mistakes.
Basically, they're to their species as Captain America is to humanity (peak and perfect physical condition). All of the alien forms present in the Omnitrix are actually the optimal genetic specimen of their species.
The device created by Azmuth that Ben Tennyson uses to transform into different aliens.