Game concept
War Contribution
The game also links it as Contribution, Contribute.
From the game's encyclopedia
Rulers who are helping their Allies in a War gain War Contribution depending on the number and size of Battles they fight in, as well as the number of Holdings they manage to Occupy.
After a war ends, each Contributor is rewarded in relation to their Contribution. The exact spoils depend on the war's Casus Belli, but is typically Gold, Prestige or Piety.
Note that Contribution increases even from losing Battles.
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.
War Contribution is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Rulers who are helping their Allies in a War gain War Contribution depending on the number and size of Battles they fight in, as well as the number of Holdings they manage to Occupy." The game also links it as Contribution and Contribute. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.