What is "clock source"?

I have an Edirol FA-66 external audio interface. In Apple's Audio MIDI Setup app, it offers me the option to have the Clock Source either from the Mac, or from the Device.

I have no idea what this means and which might be preferable.

However, it's all a bit academic, as changing to "Device" seems to crash the audio system and it restarts with "Mac" again. So I'm wondering why.