Game concept · All Under Heaven
House Bloc
The game also links it as House Blocs, House Bloc's, Bloc, Blocs, Bloc's.
From the game's encyclopedia
A House Bloc is an defensive alliance of Houses within a Realm. Any Member House who is attacked will be defended by all other Member Houses.
House Heads decide whether to become Bloc Member Houses, with all ruling members of their House in the Realm following them.
Blocs always have a Leading House, whose House Head may expel other members, is the favored Ruling Regent of all members, and whose House Aspiration determines the bonuses granted by higher levels of Cohesion.
Only Leading House's House Head may create Factions against the Top Liege, and all Bloc Member Houses members will join all Factions they create.
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.
House Bloc is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "A House Bloc is an defensive alliance of Houses within a Realm." It covers Bloc Cohesion, Bloc Member House, Bloc Member Ruler, Leading House, Bloc Supremacy War and 1 more. The game also links it as House Blocs, House Bloc's, Bloc, Blocs and 1 more. 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.