Game concept
Designated Heir
From the game's encyclopedia
The Designated Heir takes priority when deciding who becomes the Realm Heir.
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.
Designated Heir is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Designated Heir takes priority when deciding who becomes the Realm Heir." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
for a guide, post or blog
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.