Game concept
Title Allegiance
From the game's encyclopedia
Vassals can owe Allegiance to a specific Title rather than the person of their Liege. This can only happen if the Title has its own Succession Law. On Succession, such Vassals will become vassals of the new Title Holder.
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.
Title Allegiance is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Vassals can owe Allegiance to a specific Title rather than the person of their Liege." 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.