There is no way that netflix trusts the envelopes to have anything to do with the movies contained therein. Sometimes I will have three envelopes, all of which come from different fulfillment locations, and I never keep track of which DVDs go where. I just grab an envelope and send it back, and I've never seen a delay or heard a complaint. The only thing I can think of is that maybe it does contain info about a movie and its origingating location, and they can use the envelope to track inventory movement or something