Supreme Commander

Supreme Commander is a high 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, and are one of the highest grades for a Zealot Elite. They command Fleets of ships within the Covenant. This rank could be comparable to that of Admiral or General.

Physical Appearance
When commanding a fleet, Supreme Commanders wear violet armor with a cloak that wraps around and over the armor piece of their shoulders. However, when the Arbiter was being tried for the destruction of Halo, he wore golden Zealot armor, which could signify that Supreme Commander is one of several Zealot ranks, although this has yet to be confirmed. It is possible that the creators of the graphic novel were being creative.

Halo Graphic Novel
The particular Supreme Commander in the aforementioned text was a friend of and yet 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. Though this has not been explained, it is assumed that Supreme Commander is one of several Zealot ranks.

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