Front panel from left to right:
INPUT 1 GAIN (knob)
MIC/LINE 1 (female combi jack)
MIC/LINE 2 (female combi jack)
INPUT 2 GAIN (knob)
INPUT 2 HI-Z button (in is on)
Headphones (female jack)
There are three knobs on the top right above the headphones jack, from left to right they are:
MIX (far left monitors the Steinberg, centre monitors both Steinberg and computer, far right monitors computer/daw only)
Headphones volume
Line output volume

Rear panel from left to right, with the front of the device facing you, reaching over the top:
LINE OUTPUT 1/L 2/R (female jacks)
+48V phantom power switch, left is off, right is on (this switch is located directly above the output jacks)
MIDI In and Out (left is in, right is out)
USB2.0 (female type A connector)
POWER SOURCE switch, left is bus power from the computer, right powers via an external battery pack (necessary when connecting to an iPad)
5V DC port

A few words on power supply:
The UR22mkII is a bus-powered device. When connected via a USB cable, the power automatically turns on when the device is connected to the computer, and turns off when the device is disconnected or you shut the computer down.

Shout out to Brett for writing this up.

Last update: 29th April, 2021