United Cities Route is a World State Route where the player has allied relationship with United Cities and having initiated dialogue with Emperor Tengu or Koin that opens up the questline for UC.
The player can decide to speak with Emperor Tengu about quest matters, however, it will be usually directed to Koin instead, who will tell you which targets should and shouldn't be taken down.
Notably, the 3 main objectives that the UC targets are High Inquisitor Valtena, Boss Simion and Tinfist.