Game concept
Master Terrain
From the game's encyclopedia
A Decision unlocked from the Wandering Lifestyle tree, where you can choose a Terrain to specialize in, based on the group one chose in Designated Terrain.
This decision will give significant boons for the terrain chosen, giving bonuses such as increased Advantage, improved Battle Roll, reducing Attrition, and increased Prestige and Fame from Battle.
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.
Master 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 to specialize in, based on the group one chose in Designated Terrain." 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.