Q: What is the difference between a MIDI output and a MIDI thru?
A: A MIDI output sends out data that was generated within the device, by playing its keys, turning a knob, moving a performance controller, or operating its internal sequencer.
A MIDI thru sends a copy of the data that is entering a device back out, so that it can be routed to additional devices. A MIDI thru does not carry any information that was generated within the device.