Game concept
Valuable Prisoner
The game also links it as Valuable Prisoners.
From the game's encyclopedia
A Valuable Prisoner is a Prisoner who provides War Score when captured during a War.
For a person to be deemed a Valuable Prisoner, they have to be third in line or higher to inherit their war leader's primary title.
Valuable Prisoners are released when the war ends, unlike Prisoners of War.
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.
Valuable Prisoner is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Valuable Prisoner is a Prisoner who provides War Score when captured during a War." The game also links it as Valuable Prisoners. 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.