Game concept · The Fate of Iberia
Military Assistance
The game also links it as Assisting Militarily.
From the game's encyclopedia
This is a special type of Interaction available to Involved Rulers of the Iberian Struggle, allowing them to join other Involved Ruler's Wars in exchange for Gold.
Payment is made on conclusion of the War, whatever the outcome, as long as the contractor made a War Contribution of at least 100.
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.
Military Assistance is an entry of Crusader Kings III's in-game encyclopedia (The Fate of Iberia). The game explains it: "This is a special type of Interaction available to Involved Rulers of the Iberian Struggle, allowing them to join other Involved Ruler's Wars in exchange for Gold." The game also links it as Assisting Militarily. Patch 1.20.1.2, from game/common/game_concepts/03_fp2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/03_fp2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.