Game concept
Power Sharing
The game also links it as Share Power, Sharing Power.
From the game's encyclopedia
Power Sharing occurs when a Ruler shares some authority with one of their Vassals or Courtiers, who becomes their Diarch.
The Ruler who shares their power may set Mandates for their Diarch, while the Diarch may use special Borrowed Powers.
The Scales of Power measure whether the Ruler or the Diarch has the most authority.
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.
Power Sharing is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Power Sharing occurs when a Ruler shares some authority with one of their Vassals or Courtiers, who becomes their Diarch." It is part of Power Sharing. It covers Strife, Power Sharing, Diarch, Regency, Borrowed Power and 5 more. The game also links it as Share Power and Sharing Power. Patch 1.20.1.2, from game/common/game_concepts/04_ep2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/04_ep2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.