It just means there's no physical electrical connection between the devices Doug.Sounds interesting! Completely over my head though
I started by connecting the digital board of my RPi to the Cyrus amp with a coax cable.
But there were random sound dropouts - making me think the Cyrus had a problem.
But the symptom was more noticeable when switching unrelated mains electrics.
Swapped to using an optical link - no electrical connection - no further problem.
Doubt there's any problem using coax in the majority of cases - don't know if you've ever tried coax between PC and CDP, but I'd say your optical is the best choice.