Game concept
Top Liege
The game also links it as Top Lieges, Top Liege's.
From the game's encyclopedia
The 'Top Liege' is the Liege at the top of the Vassal hierarchy.
They are fully Independent Rulers and answer to no mortal.
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.
Top Liege is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The 'Top Liege' is the Liege at the top of the Vassal hierarchy." It is part of Liege. The game also links it as Top Lieges and Top Liege's. 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.