Game concept · All Under Heaven
Bloc Cohesion
The game also links it as Cohesion.
From the game's encyclopedia
Cohesion measures the unity of a House Bloc, with higher levels granting increasing benefits to members of Bloc Member Houses, based on the Leading House's House Aspiration.
The main source is House Relations between Bloc Member Houses, which can affect Cohesion positively and negatively. Additionally, the Bloc's overall size or mismatched House Aspirations or Government of members can reduce Cohesion.
Maintaining a level of Bloc Cohesion will bestow various bonuses on Member Houses, based on the Aspiration of the Leading House.
Upon reaching 0 Cohesion a House Bloc will automatically dissolve.
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.
Bloc Cohesion is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "Cohesion measures the unity of a House Bloc, with higher levels granting increasing benefits to members of Bloc Member Houses, based on the Leading House's House Aspiration." It is part of House Bloc. The game also links it as Cohesion. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.