Game concept
Succession Law
The game also links it as Succession Laws.
From the game's encyclopedia
Every Ruler has a main Succession Law that determines what happens to their Titles on Succession.
In addition, an individual Title can sometimes have its own Title Succession Law that takes precedence over the default one. For example, the Holy Roman Empire might be Elective even though the emperor has Partition as main Succession Law.
You can only change Succession Law when at the appropriate level of Crown Authority.
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.
Succession Law is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Every Ruler has a main Succession Law that determines what happens to their Titles on Succession." It is part of Law. The game also links it as Succession Laws. 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.