From the game's encyclopedia
Unruly Nomads is the lowest level of Dominance. At this level, Migration can only target De Jure County Titles within the The Great Steppe and Characters cannot be Imprisoned.
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.
Unruly Nomads is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Unruly Nomads is the lowest level of Dominance." It is part of Dominance. 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.