Game concept
Marshal
The game also links it as Marshals.
From the game's encyclopedia
The Marshal is your chief Commander and the Councillor in charge of military matters in general.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Marshal is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Marshal is your chief Commander and the Councillor in charge of military matters in general." It is part of Councillor. The game also links it as Marshals. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.