Game concept
Core
From the game's encyclopedia
A Core Tenet Status means the Rite is actively promoting this Tenet or Doctrine as one of its defining practices. All adherents of the Rite are subject to its effects.
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.
Core is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Core Tenet Status means the Rite is actively promoting this Tenet or Doctrine as one of its defining practices." It is part of Tenet Status. 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.