usb midi host = the large usb port
usb midi device = micro usb port ... and assumes you are plugging into a usb host.
(this is 'standard' usb class compliant midi terminology)
simply put, if your wanting to send midi to a midi device , like a synth/sequencer etc, then your want the large port. (so this is probably the case )
if you Axoloti to act as a device, to a computer (pc/mac etc) , then the computer is the 'host'
gpio to midi, easy stuff... take the gpio/in pipe it to a midi/out note or cc (value inlet)... and then use a change object to the trig inlet on the midi/out object