Game concept
Diarchy Veto
The game also links it as Veto.
From the game's encyclopedia
When a Diarch uses their Borrowed Powers, their Liege may use their Diarchy Veto to stop them, at the cost of Prestige
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.
Diarchy Veto is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "When a Diarch uses their Borrowed Powers, their Liege may use their Diarchy Veto to stop them, at the cost of Prestige" It is part of Regency. The game also links it as Veto. 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.