Game concept · All Under Heaven
Ruling Regent
The game also links it as Ruling Regency, Ruling Regents.
From the game's encyclopedia
Ruling Regent is the general term for any Top Liege who rules de facto on behalf of a Ceremonial Monarch, such as the Japanese Ten'nō.
The name of the Ruling Regent's position may change based on the current political circumstances; in Japan they may be referred to as Kampaku or Shōgun.
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.
Ruling Regent is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "Ruling Regent is the general term for any Top Liege who rules de facto on behalf of a Ceremonial Monarch, such as the Japanese Ten'nō." It covers Kampaku, Shōgun, Cloistered Emperor and Restored Emperor. The game also links it as Ruling Regency and Ruling Regents. 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.