Game concept
Domain Fertility
From the game's encyclopedia
Domain Fertility is the total County Fertility of all your held Counties.
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.
Domain Fertility is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Domain Fertility is the total County Fertility of all your held Counties." It is part of County Fertility. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
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Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.