Game concept
De Jure Partition
The game also links it as De Jure Partitioned.
From the game's encyclopedia
Realms that have been De Jure Partitioned have been split into two same-Tier Titles, but can still be reunited easily.
Either Title can Drift Completely Controlled Duchies from the other Title into its De Jure instantly via Decision.
If either Title is ever destroyed or loses the last of its De Jure Counties, then the remaining Title absorbs all of its De Jure.
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.
De Jure Partition is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Realms that have been De Jure Partitioned have been split into two same-Tier Titles, but can still be reunited easily." The game also links it as De Jure Partitioned. 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.