Game concept
Men-at-Arms Counter
The game also links it as Counter, Counters.
From the game's encyclopedia
Most Men-at-Arms are able to Counter another type of Men-at-Arms.
When a Regiment is Countered, its effectiveness during a Battle is decreased. The more a Regiment is outnumbered by its Counter, the higher the penalty.
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.
Men-at-Arms Counter is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Most Men-at-Arms are able to Counter another type of Men-at-Arms." It is part of Men-at-Arms. The game also links it as Counter and Counters. 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.