What makes the Pontifical Mission Societies unique compared to other mission organizations?

According to the teaching of Vatican Council II, the Pontifical Mission Societies are institutions of the Universal Church and of each local church.  Therefore, unlike any other mission organization, the Pontifical Mission Societies are both Pontifical and Episcopal in nature.  As such, the Pontifical Mission Societies are the primary mission organization of the Universal Church that encourages the work of mission, prayer and financial support for evangelization of the Church throughout the world.