Game concept
Designated Terrain
From the game's encyclopedia
A Decision unlocked from the Wandering Lifestyle tree, where you can choose a Terrain group to specialize in. Certain Perks will provide different boons to the terrain group chosen.
Terrain groups one can choose from:
• 
Forests / 
Jungles / 
Taiga / 
Wetlands
• 
Mountains / 
Hills / 
Desert Mountains
• 
Plains / 
Steppe / 
Farmlands / 
Floodplains
• 
Deserts / 
Drylands / 
Oasis
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 Terrain is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Decision unlocked from the Wandering Lifestyle tree, where you can choose a Terrain group to specialize in." It is part of Decision. Patch 1.20.1.2, from game/common/game_concepts/01_bp3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/01_bp3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.