Game concept
Fallback Heir
From the game's encyclopedia
The Fallback Heir becomes the Realm Heir for the targeted Title when the regular line of Succession is empty.
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.
Fallback Heir is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Fallback Heir becomes the Realm Heir for the targeted Title when the regular line of Succession is empty." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
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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.