From Halopedia, the Halo wiki
Obedientiaries (Sangheili: lesser chosen, charged with glory) are a rate of senior personnel employed in the Covenant military - serving as the rough equivalent to the concept of commissioned officers in the United Nations Space Command and Colonial Military Authority. Obedientiaries are veteran warriors who have begun to step foot on the path of command, and are thus expected to experience every facet of warfare. A part of this selection process involves those warriors who are able to temper their bloodlust and survive their first battles - with their experiential process expected to see them take on missions requiring cunning and courage in equal measure.
Obedientiary is not an explicit rank, but an overall grouping for a number of ranks and roles found throughout the Covenant and its remnants. The ranks of Major, Officer and Commander all fall within this grouping, with the Enforcers of the Banished modelled after the Covenant's obedientiaries to fulfil a similar role. Obedientiaries may be expected to organise lesser warriors such as Minors and thralls, though they will still nonetheless seek honour and glory in a similar manner as any younger warrior. They generally sport more ornamental harnesses usually in a red (or similar) colouration to denote their status - and will often close with the enemy and engage their forces directly on the battlefield in comparison to the UNSC, who expect their officers to lead from a more dispassionate view. These tendencies, alongside their skill-at-arms, mean that their distinct visual aesthetic is the only way human forces are able to distinguish Obedientiaries from their lessers.
As part of their requirements to experience all aspects of war, Obedientiaries may fulfil the duty of officers aboard smaller Covenant starships such as intrusion corvettes. Pilots of Covenant space fighters and dropships such as Seraphs and Spirits were also drawn from the ranks of the obedientiaries.
In medieval Europe, an obedientiary was a lesser position in a monastery, appointed by a superior. The name is drawn from the latin word obedientiarius, meaning someone in an obedient or subordinate position.