"Downmixing" refers to generating a stereo track from a multichannel track. You find that in DVD-Video discs that have a Dolby Digital 5.1 track and no stereo track. In those cases, DD can create a 2 channel track on the fly.

Similarly, in DVD-Audio discs a downmix can be greated in the DVD-Video zone by either a DD downmix or by the downmix parameters that are part of Meridian's MLP compression system.

In the case of Super Audio CD (SACD), the record labels create a separate stereo mix at the studio vs. having the player or processor downmix to 2 channel.