Game concept
Fallback Heir Title
From the game's encyclopedia
The Holder of the Fallback Heir Title 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 Title is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Holder of the Fallback Heir Title 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.
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.