Game concept
Situation Phase
The game also links it as Situation Phases.
From the game's encyclopedia
Each Situation Phase has its own modifiers and conditions that affect the involved characters.
Some Situation Phases are triggered by specific conditions, while others simply follow the passage of time.
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.
Situation Phase is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Each Situation Phase has its own modifiers and conditions that affect the involved characters." It is part of Situation. The game also links it as Situation Phases. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.