Game concept
Capital Duchy
From the game's encyclopedia
The Capital Duchy Modifier increases the amount of Levies you get out of all Holdings you hold within the same De Jure Duchy as your Capital. It also gives a bonus to all Stationed Men-at-Arms. For this modifier to be active, you need to personally hold the associated Duchy.
This modifier does not stack with Realm Capital bonus.
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.
Capital Duchy is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Capital Duchy Modifier increases the amount of Levies you get out of all Holdings you hold within the same De Jure Duchy as your Capital." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.