Game concept
Budget Base Rate
The game also links it as Base Rate.
From the game's encyclopedia
The amount a standard recipient of a Budget Category receives. Recipients may receive more or less than the base rate depending on their Rank, Merit Rank, or other modifiers.
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.
Budget Base Rate is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The amount a standard recipient of a Budget Category receives." It is part of Treasury Budgeting. The game also links it as Base Rate. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.