This application is a host for music devices audio units. Music devices are specialized OS X audio units that generate sounds in response to some control protocol (MIDI in our case), like virtual synthesizers.
Usage: Create a document of the desired type and select the desired MIDI input.
To connect the synth input to another application, you can use the IAC (inter application communication) bus. To create an IAC bus, refer to the Audio MIDI Setup apple utility help (Applications/Utility/Audio MIDI Setup).
You can also specify which MIDI channels are processed (default is all channels).