Game concept
Legitimacy Level
The game also links it as Legitimacy Levels, Legitimacy Gain.
From the game's encyclopedia
Legitimacy Levels can be gained through various means: winning Wars, releasing Valuable Prisoners, creating Legends, holding Court, or attending Activities.
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.
Legitimacy Level is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Legitimacy Levels can be gained through various means: winning Wars, releasing Valuable Prisoners, creating Legends, holding Court, or attending Activities." It is part of Legitimacy. The game also links it as Legitimacy Levels and Legitimacy Gain. Patch 1.20.1.2, from game/common/game_concepts/06_ce1_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/06_ce1_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.