Duplicating on demand on to a CD-R as opposed to a replicated i.e. pressed disc and with a digital on body print on the disc as opposed to silk screen/or offset printing for the on disc artwork that pressed discs have.
Interesting concept in keeping deep back catalogue in print - also interesting in as much as to be able to do this the major labels would have to allow Amazon to keep the albums on their own digital vault i.e. server.