...do they sell SPDIF to TOSLINK audio cables?
There are also adapters which convert A/D for S/PDIF inputs and/or D/A for the outputs, enabling connection to other audio gear with only analog ins/outs.
So, the analog outs are labeled as "Mix" in Cubase and "DAW" is the SPDIF out?
What about the "FX Bus"?
The outs seen by Cubase are the mix busses 1-3. These can be output onto either physical output pairs A,B or C in alternate mode (meaning you can listen to a given mix on any of three separate monitor pairs- one at a time) or all three mix busses can be output onto separate A B C pairs in independent mode (thus enabling 6ch surround - i.e. any track can be individually routed to any of the 6 separate physical outputs).
The output of the S/PDIF can be selected from any of the Mix bus pairs or the DAW (9,10) out pair, selectable from the control panel for the UR28M and is a good choice for external devices.
The FX bus refers to the internal Reverb Bus.