Game concept · All Under Heaven
Bloc Member Ruler
The game also links it as Bloc Member Rulers, Bloc Ruler, Bloc Rulers.
From the game's encyclopedia
A Bloc Member Ruler is a Ruler who is a member of a House Bloc.
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 Member Ruler is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "A Bloc Member Ruler is a Ruler who is a member of a House Bloc." It is part of House Bloc. The game also links it as Bloc Member Rulers, Bloc Ruler and Bloc Rulers. 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.