Game concept
Manually Reinforce
The game also links it as Reinforcements.
From the game's encyclopedia
Adventurers and Nomads may recruit Men-at-Arms, but they do not reinforce automatically as Landed Ruler's do. Instead, recruits must be enticed using interactions.
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.
Manually Reinforce is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Adventurers and Nomads may recruit Men-at-Arms, but they do not reinforce automatically as Landed Ruler's do." It is part of Interaction. It covers Siege Replenishment. The game also links it as Reinforcements. 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.