[LAU] About Digital I/O

[Gabriel]

Hey everyone !! I have a question here about my usb audio interface. For 
reference it is a UA 55 Quad Capture. But I'm specifically asking about 
the digital input it has. How can I use that digital input ( COAXIAL ) ? 
Can I "expand" my audio interface with two more channels coming from 
another usb interface or any outboard gear, and if that is doable, how 
can I do it ? And the outputs, what are they good for ? How can I use 
them to improve my recording workspace and gear ?