Game concept
Diarch Aptitude
From the game's encyclopedia
Diarch Aptitude governs how well a Diarch achieves each Mandate set by their Liege.
Both Skills and Personality Traits play a role. For instance, a Greedy Diarch tasked with the 'Fill Coffers' Mandate may not send as much Gold to their Liege.
Diarchs are well-suited to a Mandate when they have around 40+ Aptitude for it.
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.
Diarch Aptitude is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Diarch Aptitude governs how well a Diarch achieves each Mandate set by their Liege." It is part of Power Sharing. 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.