From the game's encyclopedia
A Casus Belli used by a Challenger Head of Faith or their sponsor against the legitimate Head of Faith of their Faith. Victory installs the Challenger Head of Faith as Head of Faith and makes their Rite the Mainline Rite. A sponsor who wins the war and outranks their Challenger Head of Faith also takes them as a Vassal.
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.
Challenge Head of Faith is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Casus Belli used by a Challenger Head of Faith or their sponsor against the legitimate Head of Faith of their Faith." It is part of Casus Belli. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/pam_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.