Or so I thought 
It wasn’t until the last few years that I began actually using digital audio connections on my audio interfaces, not knowing anything about them and how they could be used when I bought them a long time ago.
Today, my continued research & experimentation may have turned around my previous understanding of digital audio signals concerning their inherent clock. Using multiple audio interfaces under a single driver, with their digital s/pdif audio connections…
I had assumed for quite some time that setting the ‘clock master’ on a digital audio device meant to select ‘s/pdif’ … and the others to the sample rate of say, 44.1 Khz as 'clock slaves’.
I believe I was wrong on this, and that the device to be the ‘master clock’ should be set to your preferred sample rate, of say 44.1 Khz, and all other devices meant as ‘clock slaves’ should be selected as ‘s/pdif’.
Obviously, if you’re only using two digital audio devices, one can be set to ‘s/pdif’ and the other to say, ‘44.1 Khz’ and you’ll be fine, but start using three (or more) digital devices and it’s not ‘one to one’ anymore, but now it’s ‘one to two’, or ‘two to one’. We now have one that’s the right way, and another that’s the wrong way 
At 1st, various combination of the above seems to work, but then get a little ‘quirky’ after a while. The new way seems less quirky. I’m assuming that with other different digital audio connections, things are very similar, or near the same and may be labeled differently of course.
Was I right the 1st time, and now wrong? Thoughts? Corrections? Additions?
SOLVED it,
Seems my new understanding is correct… that the ‘master clock’ should be set to the actual preferred ‘sample rate’. In my own audio cards settings (Aardvark Q10’s), for the master clock, it should be set to ‘Int 44.1’ all other Q10’s should be set to s/pdif to ‘follow’.I hadn’t realized that ‘Int’ means ‘Internal’, for Internal Clock.
Oddly enough when I had 2 of the 3 Q10’s set to ‘Int 44.1’ and the 3rd Q10 set to s/pdif, having set two as master clocks and one set to s/pdif to follow, it all seems fine either way, as well as setting all THREE to s/pdif. but now that I know, I may as well keep the settings correct.