Game concept
Clerical Region Capital
The game also links it as Clerical Region Capitals.
From the game's encyclopedia
The Clerical Region Capital is the County a Clerical Region was founded upon with an Ecclesiastical Estate, serving as the seat of the Clerical Title that governs the region. Unlike a De Jure Capital, it is fixed to the region and cannot be relocated.
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.
Clerical Region Capital is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Clerical Region Capital is the County a Clerical Region was founded upon with an Ecclesiastical Estate, serving as the seat of the Clerical Title that governs the region." It is part of Clerical Region. The game also links it as Clerical Region Capitals. 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.