"Name of one of the seven great Parthian families; they ruled Seistan as vassals of the Arsacids, and held certain hereditary rights and functions, especially those of crowning the king at the coronation ceremony and of military command in the field" (C.B.R. Pelling in OCD4 s.v., with detail on the best-known Suren, who defeated the Parthian invasion of M. Crassus in 53 BCE).