Game concept
Confederate
The game also links it as Confederates.
From the game's encyclopedia
A Ruler who is in a Confederation. They will automatically join the Defensive Wars of their fellow Confederates, and are restricted from attacking or Raiding them.
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.
Confederate is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Ruler who is in a Confederation." It is part of Confederation. The game also links it as Confederates. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.