You would want to use the back side (USB-A). The front side is for connecting to your PC if you want to run the Auracle X software to configure the MIDI routing. Definitely get familiar with that software though, as it's important in order to ensure things are routed the way you want. There's lots of pretty amazing things you can do with the mioXM including connecting over WiFi with rtpMIDI (on an iPad or using rtpMIDI on Windows https://www.tobias-erichsen.de/software/rtpmidi.html) or connecting over bluetooth with devices that don't normally support bluetooth if you use something like a WIDI Jack on the keyboard and a WIDI Master on the mioXM (https://zubersoft.com/mobilesheets/accessories/ has the WIDI Master listed).
Mike
Mike