The Imperial Admiral rank is known to be the highest Covenant naval rank. Imperial Admirals lead whole fleets.
The Supreme Commander is the Covenant Commander of a rather large or powerful fleet.
The Field Marshal falls under the Zealot Class, but appears to outrank regular Zealots. Besides leading large-scale operations, Field Marshals are known to lead small strikes composed of Zealot Officers.
Fleet Masters command armadas.
Field Masters lead armies and are in charge of everything within their army such as Air Support, Armor and it's members.
Ship Masters command Ships including their own Ship.
Employed by the Ministry of Fervent Intercession, Zealot Elites serve as the Admirals and Generals of the Covenant Military; Commanders of Battalions, Ships and Fleets.
Generals are the highest ranking military personnel employed by the Covenant among their groundside forces. They lead hundreds and even thousands.
Ultras are the Covenant's Colonels and Captains, they command individual operations.
Majors are veteran Elites who are equivalent to UNSC Lieutenants. Overall they are more agile, aggressive and accurate than Minors. A single Major is capable of surviving a whole squad of UNSC Marines. They can lead Minors, Grunts and Jackals.
Minors are the most common but least experienced. Despite this, Minors are extremly skilled warriors. Minors are equivalent to UNSC Sergeants and are often seen leading squads of Grunts.