Game concept
Head of Faith Succession Law
The game also links it as Head of Faith Law, Head of Faith Laws.
From the game's encyclopedia
Rulers who are also Heads of Faith have an additional realm Succession law, requiring that their Heir be of the same Faith as them. This can never be changed, and Head of Faith Titles cannot have separate Title Succession. The only way to remove this type of realm law is for an external force to destroy the Head of Faith Title.
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.
Head of Faith Succession Law is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Rulers who are also Heads of Faith have an additional realm Succession law, requiring that their Heir be of the same Faith as them." It is part of Law. The game also links it as Head of Faith Law and Head of Faith 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.