Game concept
Partition Share
From the game's encyclopedia
Under any Succession Law with multiple Realm Heirs, a Character's Partition Share is which Titles that particular Heir will inherit upon Succession. Usually, this means dividing between the children of a Ruler, or, rarely, their siblings.
Typically, depending on what type of Succession is in place, Titles may be divided evenly between separate Realms, evenly within the same Realm, or given mostly to the main Heir while dividing the remainder evenly between any other Heirs.
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.
Partition Share is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Under any Succession Law with multiple Realm Heirs, a Character's Partition Share is which Titles that particular Heir will inherit upon Succession." 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.