Supreme Commander

Supreme Commander is a Covenant Sangheili (Elite) rank.

Rank
In the known Elite rank structure, the Supreme Commanders are the fourth-highest ranking Elites, beneath only the Councilors, Arbiters, and Imperial Admirals.

Supreme Commanders are probably the Covenant equivalent of Vice Admirals and Admirals/Lieutenant Generals and Generals, although this has not been confirmed.

Physical
Supreme Commander armor is depicted as clad in bright violet armor, with a long cloak that wraps around his shoulders.

Halo Graphic Novel
The particular Supreme Commander in the aforementioned text was a friend and superior to Rtas 'Vadumee, and would later become the Arbiter. Interestingly enough, this same Commander wears gold Zealot armor in his trial at High Charity. This change in armor could be assumed the outcome of simply not being a Supreme Commander anymore. The future Arbiter was still a Fleet Master or Ship Master prior to and during his trial before the High Council and thus still wore the armor of his rank. So far, only one has been seen.

During the Civil War of the Covenant, it is possible the Brutes usurped these positions from the Elites.