const CFStringRef kMIDIPropertySingleRealtimeEntity;
device property, integer
Some MIDI interfaces cannot route MIDI realtime messages to individual
outputs; they are broadcast. On such devices the inverse is usually
also true -- incoming realtime messages cannot be identified as originating
from any particular source.
When this property is set on a driver device, it signifies the 0-based index of
the entity on which incoming realtime messages from the device will appear
to have originated from.
New for CoreMIDI 1.3.