A matrix decoder attempts to extract multiple audio channels from a stereo pair. This extra channel content can be placed in the matrix by a purposeful encoding process, or as a byproduct of effects such as reverb. The SU-27 takes a stereo input, and processes it to create 3, 4, or 5 channels of output.
The additional channels created by the SU-27 can either be directly wired to the hardware output jacks in Reason, for use with multi-channel audio interfaces. Or exported to disk a series of mono or stereo tracks to be combined together with one of several freely available utilities.