Game concept
House Succession Law
From the game's encyclopedia
In Clan Realms the current House Unity Level dictates which Succession Law is used.
Clans can switch to a Single Realm Heir Succession Law, which will prevent House Unity from changing it, but doing so is much harder than normal.
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.
House Succession Law is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "In Clan Realms the current House Unity Level dictates which Succession Law is used." Patch 1.20.1.2, from game/common/game_concepts/03_fp3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/03_fp3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.