These Grunts serve as field officers to their comrades, and as this rank is the highest in the Grunts race, the Ultras have no control whatsoever over the other Covenant races (except for Kig Yar/Jackal). Ultras were heavy armor and throw Plasma Grenades more than using firearms.
Similiar to a UNSC as a Lance Corporal, they are more experienced than Minors. They command several Grunt Minors and are slightly more skilled in combat and courageous.
Grunt Minors are equivalent of a UNSC Private or Private First Class. Minors are often the least dangerous, most cowardly, and inaccurate marksmen of the Covenant. When their commanding officer is killed, Minors retreat from the enemy.