Imagine back to a time when the only MIDI controllers were
- Keyboards with nothing but keys, mod and pitch wheel
- MIDI fader/knob boxes, with no keys at all
“Music Data” and “Control Data” are meta-data tagging to tell Ardour that a device fits into one of the above categories. This means that Ardour will not auto-connect your MIDI tracks’ inputs to your MIDI fader/knob box, as is typically desired.
However, we now live in an era of mixed-use devices: controllers with keys, pads, knobs and buttons. It’s distinctly non-trivial to identify/decide whether such a device should be used as a “music” input, or a “control” input, or both (or possibly neither).
This is why Ardour’s Push 2 support creates a virtual MIDI port for the device that represents only the pads and this can be treated like a “music data” device, while leaving the buttons etc. as “control” (there are other options too).