An elite rank. During the games it is a punishment and they are sent on suicide missions.
Arbiter - One who has absolute power in judging or determining. In the Halo series, the Arbiter is the supreme commander of the Sangheili race, hence the title "Arbiter." The Covenant government lessened the importance of this role to allow no one person to of greater stature than the Prophets and/or the High Council.
Arbiter is not the supreme commander of anything, it is a elite is punished and used mostly to settle disputes or eliminate high priorty threats. Arbiter was purely a high elite rank until I think it was the Grunt Rebellion.