Game concept
Succession
The game also links it as Diarch Succession.
From the game's encyclopedia
Diarch Succession governs which of a Liege's direct Vassals or Courtiers is automatically made Diarch when one is needed.
This happens either because a Liege begins Power Sharing or because the current Diarch dies or is removed from office.
Different types of Power Sharing may calculate succession order differently — in a Regency, landless Characters with strong personal connections to their Liege tend to dominate.
Speaking the Liege's Language (and Court Language, if they have one) is almost always crucial.
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.
Succession is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Diarch Succession governs which of a Liege's direct Vassals or Courtiers is automatically made Diarch when one is needed." It is part of Power Sharing. The game also links it as Diarch Succession. 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.