USB C port for midi, should be enabled by default but you'll need to turn on midi through if you want that via the thru port from usb or midi in, check the settings under the global settings > midi - the manual explains this and has details on the ccs etc
https://manuals.line6.com/en/helix-stadium/live/midi
https://manuals.line6.com/en/helix-stadium/live/global-settings#globals-midi