Game concept · All Under Heaven
Mandate of Heaven
The game also links it as Mandate.
From the game's encyclopedia
The Mandate of Heaven is a special type of Legitimacy only available to Son of Heaven. It represents the monarch's right to rule, and who is considered the supreme ruler under all of heaven.
The Mandate is closely connected to the Dynastic Cycle. The Cycle affect how the reigning monarch gains Legitimacy and may also change Dynastic Cycle Era based on the monarch's level of Mandate.
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.
Mandate of Heaven is an entry of Crusader Kings III's in-game encyclopedia (All Under Heaven). The game explains it: "The Mandate of Heaven is a special type of Legitimacy only available to Son of Heaven." It is part of Legitimacy. It covers Son of Heaven. The game also links it as Mandate. Patch 1.20.1.2, from game/common/game_concepts/tgp_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/tgp_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.