Game concept
Scheme Success Chance
The game also links it as Success Chance.
From the game's encyclopedia
The Success Chance of a Scheme determines how likely it is for it to succeed when executed.
A Scheme's Success Chance will grow over time, and is based on the assigned Agents as well as an associated Skill. Success Chance is limited by Scheme Potential. No Scheme can ever exceed a 95% Success Chance, except for Contract Schemes.
Secret Schemes have a risk of being discovered by target. If that happens your Success Chance will be heavily reduced.
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.
Scheme Success Chance is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Success Chance of a Scheme determines how likely it is for it to succeed when executed." It is part of Scheme. It covers Scheme Growth and Scheme Potential. The game also links it as Success Chance. 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.