From the game's encyclopedia
A Casus Belli available to Rulers bordering a Realm governed by a Bureaucratic Government Form.
It allows the attacker to seize a neighboring Duchy that lies outside the defender's capital's De Jure hierarchy.
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.
Seize Peripheral Duchy is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Casus Belli available to Rulers bordering a Realm governed by a Bureaucratic Government Form." It is part of Casus Belli. Patch 1.20.1.2, from game/common/game_concepts/07_ep3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/07_ep3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.