so, in the list of MIDI input/output ports we have the following:
1st and last positions are clear — din is MIDI DIN, usb device port 1 refers to micro-USB port.
what are others? what can i use them for?
Usb host ports 1-4 are for the the BIG usb port, for example for connecting midi controllers directly to Axoloti, like a launch pad mini for example, or a synth that uses usb midi like Blofeld. Don't know why there are 4 of them, actually, (since there is only one USB host port and it doesnt support usb hubs). I always use just the first one, usb host port 1.
The internal ports 1+2 can be used to send midi internally for all sorts of stuff.
thanks! this is exactly what i wanted to know right now.
anyway, it's interesting why there are 4 USB host ports.
probably someone else could explain it in details.
p.s. noticed that MIDI-related stuff in general is significantly less well documented.
usb midi protocol allows for more then one midi connection per device, so a synth could potentially be 32x multitimbral but would need two usb-midi connections to control them all...
yeah ports are 'tricky', as its not often exposed to users....
they basically exist (in the usb protocol) to move us beyond 16 channels of midi imposed by midi din.
many midi users will think, 16 channels of midi is fine, but since the days of multi-timbral instrumentsand MPE! , which use multiple channels per instrument - suddenly that 16 feels not so generous 
a good example of where ports are commonly used are usb midi to din routers, where you have one usb input , and then four din out - so each din now has its own 'port' and its own 16 channels!
note: its not limited to 4 ports, its actually 16, but I decided to limit the UI for simplicity, as I knew most would not know/nor care what the ports were, so I compromised on 4
( a note for completeness, a USB hub, is not the same as a USB midi hub , the later is supported, the former not in 1.0.12)
cool! i've discovered some fun port stuff & uses last few hours, thank you for clarifying things and putting it all to order.
Sorry for the revival, but I still have some confusion about this.
I have one axoloti (A) hosting another axoloti (B).
If I wanted to send MIDI from A to B, I would use a midi out object with "host port 1"
If I wanted to send MIDI from B to A, I would use a midi out object with "device port 1"
I've tried this, and the reverse, but it doesn't seem to work for me.
MIDI in objects don't give an option for input source, so I assume they are global unless put in a subpatch with MIDI selector - which I have also tried without success.
Never mind, I just forgot to plug in a "logic/change" object before the MIDI out trig input