Game concept
Gender Law
The game also links it as Gender Laws.
From the game's encyclopedia
Every Ruler has a main Gender Law that determines who their Titles can pass to on Succession. Gender Laws also affect who can command Armies and the eligibility of having multiple Spouses or Concubines.
Available Gender Laws depend on the 'View on Gender' Doctrine of the Faith.
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.
Gender Law is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Every Ruler has a main Gender Law that determines who their Titles can pass to on Succession." It is part of Law. The game also links it as Gender 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.