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)