Game concept
Expand Dominance
The game also links it as Dominance Expansion.
From the game's encyclopedia
A special war for Nomadic Rulers for either an area within their De Jure lands, or to expand their De Jure area to new frontiers.
All Nomadic Rulers of a Duchy Tier or higher can start the war within their De Jure. On a victory they will attempt to either Vassalize the defender or force them to become a Tributary, or conquer their lands altogether if they hold land outside of the target lands of the war.
To expand their dominion to foreign lands, a Nomadic Ruler has to be of a Kingdom Tier or higher. On a victory they will still attempt to Vassalize, Tributarize, or conquer the lands, but the lands will also immediately shift De Jure to the primary title of the Nomadic ruler.
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.
Expand Dominance is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A special war for Nomadic Rulers for either an area within their De Jure lands, or to expand their De Jure area to new frontiers." It is part of War. The game also links it as Dominance Expansion. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.