From the game's encyclopedia
High Crown Authority is the second to highest level. Vassals have a -20 Opinion penalty toward their Liege.
At this level — in addition to the powers the liege gets at Limited Crown Authority — vassals under the same liege can no longer wage war against each other, the ruler can change between all available Succession Laws, and vassals' Titles cannot be inherited by rulers outside the liege's Realm.
Lastly, refusing title Revocation or vassal Retraction is considered a Crime even if the liege lacks sufficient justification.
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.
High Crown Authority is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "High Crown Authority is the second to highest level." It is part of Crown Authority. The game also links it as High. 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.