It is a sound only decoder used in conjunction with a motor controller decoder on DCC to provide sound. It must be programmed with the same address as the other decoder, or at least consisted with it. It can be mounted in the same loco, in a trailing car, or under the layout, but must be connected to the same DCC power bus.
Programming 2 decoders to work together properly is not always easy, as owners of the BLI Blue Line locomotives have found out when they added a motor controller decoder to the existing sound decoder.
I personally don't see the advantage of a two decoder setup to get sound - except perhaps for under the table. Even then, you would need a separate sound decoder for each locomotive you wanted to have sound, and a mixing panel to feed the speakers. It seems a lot easier to replace the existing decoder with a combination controller and sound unit - unless there is absolutely no room in the locomotive for the speaker and enclosure.
my thoughts, your choices
