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A Be star, in the classical sense used here, is a rapidly rotating B-type star that shows or has shown emission in Balmer series lines from circumstellar gas. Its decretion disk is approximately Keplerian rotation and differs from a young star's dust-bearing protoplanetary disk. Disk growth and loss can change the emission spectrum. The viscous disk model describes the gas after injection; it does not by itself explain how material is lifted from the stellar surface.^[ref]

A decretion disk is supplied near its inner edge and transports matter outward. Internal stresses redistribute the supplied angular momentum through a nearly orbiting flow. Classical Be stars provide an example: their gaseous circumstellar disks are modeled as viscously spreading, approximately Keplerian disks. A disk carrying an outward torque is not automatically a decretion disk; it must actually have outward mass transport.^[ref]