Game concept
Scheme Growth
The game also links it as Growth, grow.
From the game's encyclopedia
Every Phase, a Scheme's Success Chance is increased by this value, until the maximum current Potential is hit.
For instance, if a Scheme currently has 50% Success Chance, a Potential of 54%, and a Growth of 3%, then after one Phase has gone by, it'll have 53% Success Chance.
Since Growth is still capped by Potential, after the next phase, the Scheme would only have 54% Success Chance.
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 Growth is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Every Phase, a Scheme's Success Chance is increased by this value, until the maximum current Potential is hit." It is part of Scheme Success Chance. The game also links it as Growth and grow. 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.