Game concept
Provincial Army
The game also links it as Provincial Armies, Thematic Army, Thematic Armies.
From the game's encyclopedia
A Provincial Army is an army recruited and maintained by a Governor in a Realm with the Bureaucratic Government.
They are exclusively made up of Title Men-at-Arms. The effectiveness of Men-at-Arms are affected by a Governor's Governor Efficiency.
The Liege can spend Influence to gain control of Armies.
Governors may borrow troops from their peers under certain circumstances, such as being Allies or using a Hook.
A borrowed Army is returned if the Character who borrowed them dies or when they end all of their ongoing Wars. Armies can also be returned manually.
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.
Provincial Army is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Provincial Army is an army recruited and maintained by a Governor in a Realm with the Bureaucratic Government." It is part of Administrative. The game also links it as Provincial Armies, Thematic Army and Thematic Armies. Patch 1.20.1.2, from game/common/game_concepts/07_ep3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/07_ep3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.