Marshal was the title of the highest uniformed officer in the Earth Federation. They were the commander-in-chief of all military forces, superceding the CNC's of Earth Fleet, Tech Infantry, and Light Infantry. The office was established under the new constitution after the 1st Civil War, as a check on the power of the Grand Council, as the Marshal was directly elected (and could be removed) by the Senate. Later on, they became a de facto member of the Grand Council.
However, the role of Marshal was never quite realized for what it was intended. Two marshals were convicted of crimes under the military code (including Lwan Eddington), three were removed, and one died. The last powerful marshal was Sarah Dunmeyer. After the 2nd Civil War and the defection of the Tech Infantry, the office of marshal was left vacant under the administration of Arthur Clarke.