Game concept
Ducal Conquest Casus Belli
The game also links it as Ducal Conquest.
From the game's encyclopedia
A Casus Belli that allows the seizure of Counties required to create an Uncreated Title of Duchy rank.
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.
Ducal Conquest Casus Belli is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Casus Belli that allows the seizure of Counties required to create an Uncreated Title of Duchy rank." It is part of Casus Belli. The game also links it as Ducal Conquest. 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.