Hi neo36uk,
User channel is used principally by CONTROL MODE. You need to set it in the MIDI Control Center (MCC).
MIDI protocol allows 16 channels. You can decide to set a specific channel as "user channel".
When you set your step buttons, knobs or pads to send midi information, you must choose a channel. This channel is set between 1 and 16.
If you set all your CONTROL MODE set-up to send MIDI over channel 1, then, if afterwards you want to change to channel 2, you have to change each and every knobs, steps and pads to channel 2, one by one (16 knobs + 16 steps + 16 pads = 48 changes needed). If you set every one to user channel, this way you can change the user channel affectation in the MCC from 1 to 2 and that's it.
Regards,
Y.