SACD Multi-Channel down-mixing?

Hi, There's been past discussions on this forum about Marantz SACD multi-channel players that "down-mix" or "down-sample" the signal through its HDMI output. Can someone explain exactly what that means? What is down- mixing or down-sampling an SACD signal? I'm assuming both terms mean the exact same thing. Thanks.
Down-mixing is combining the 5.1 multi-channel program to accommodate a two-channel system.

Down sampling is taking a DSD or 192/24 data-stream and converting it to 44.1/16 so your current DAC can play it.
Thanks. I since found out that there is a setting in the set up menu of the Marantz UD5005 for down-sampling so it can be activated or turned off. This player outputs DSD through its HDMI circuit and does not down-mix it.