Re-reading the manual of ASIO4ALL I am actually not sure about this at all. It rather looks like ASIO4ALL doesn’t give a damn and if it works you are just lucky. USB devices can sync themselves to the computer’s clock but, of course, can also have their own little quartz-crystal. It seems to be up to the manufacturer of each device what they offer. I don’t elaborate on the sample-rate as it is always the user’s obligation to match the sample-rates of all devices.
I am not sure how clock-sync can be achieved when you have a USB and a non-USB (PCIe) device.
If anybody can recommend a good read on this topic, kindly post it.
What puzzles me a bit is this option:
as I was under the impression that Cubase always syncs its audio engine to the selected ASIO device and thusly always is externally clocked. So what is this good for?