Game concept
Situation Region
The game also links it as Situation Regions.
From the game's encyclopedia
Situations may have more than one region, each of them with independent Situation Phases.
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.
Situation Region is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Situations may have more than one region, each of them with independent Situation Phases." It is part of Situation. The game also links it as Situation Regions. 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.