Game concept
Stationed Regiment
The game also links it as Station Regiment, Stationed, Station, Stationed Holding.
From the game's encyclopedia
A Men-at-Arms Regiment receives bonuses to its effectiveness by being Stationed in a Holding.
Buildings can further increase these bonuses — sometimes for any type of Men-at-Arms, but most commonly for one or two. For example, Small Stables provides a bonus to Stationed Light Cavalry and Heavy Cavalry.
You can Station at any time except during War, or if the Holding is Occupied. If a Holding is lost, any Stationed Men-at-Arms will be unstationed.
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.
Stationed Regiment is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Men-at-Arms Regiment receives bonuses to its effectiveness by being Stationed in a Holding." It is part of Regiment. The game also links it as Station Regiment, Stationed, Station and Stationed Holding. 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.