The USB type-B connector is attached to the RPi USB type-C connector. It exposes the Zynthian’s “Host Computer” MIDI port. Connecting this to a computer’s USB type-B (or type-C) port should present the computer as a MIDI port to Zynthian and Zynthian as a MIDI port to the computer. This is known as “gadget mode” or “On The Go” (OTG).
We do not support audio over this link (yet), only MIDI. We may investigate other options in the future. (Audio was not possible when we first implemented this feature and I don’t know if it is yet but either way, audio presents substantial challenges due to the way jackd work.)
In Oram this port will appear in the MIDI input/output lists only when the port is connected to a computer that has recognised it as a MIDI device. It will appear as “HOST USB MIDI”.