From Halopedia, the Halo wiki
Field Master is a title given to a high-ranking Generals in the Covenant military. The title is one of exclusive Sangheili domain due to the Sangheili command over the Covenant's now-defunct armed forces.
Field Masters consist of select Generals, tasked with taking charge of temporarily-assembled large military groups such as warhosts. They operate at a similar command level to the aforementioned Generals, or Shipmasters. They are typically signified by their golden harnesses.
The title of Field Master was continued by several remnant factions of the defunct empire, including Merg Vol's Covenant and Jul 'Mdama's Covenant. Merg Vol deployed a number of Field Masters during his invasion of Draetheus V in 2555, using the unique magnetic signatures of the planet to hide their location. Ultimately, they were hunted down and assassinated by SPARTAN-IV Sarah Palmer. These Field Masters were outfitted with Nakata'vho-pattern plasma repeaters. The Battle of Installation 03 saw Jul 'Mdama deploy several Field Masters armed with M'tara-pattern focus rifles onto the surface of Gamma Halo.
List of appearances