From the game's encyclopedia
Most Titles of are considered part of a higher-Tier Title De Jure ('by law').
For example, a Ruler may be King of England, but their Realm does not yet contain all the lands that are legally considered to be part of England.
The Holder of a Duchy, Kingdom, or Empire always has a Casus Belli to seize control of its De Jure constituent titles.
Vassals will also decide who their Rightful Liege is based on what Title their Primary Title is De Jure a part of.
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 is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Most Titles of are considered part of a higher-Tier Title De Jure ('by law')." It covers De Jure Drift. 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.