'Soundcard' means you select that device as the Driver in BiaB/RB/PT
So your computer plays back to (& records from) it.
So you would record your new tracks into the mixer (soundcard) physical connection and it would be sent via USB to the software as a digital signal from that device. And playback would be able to be routed to it also.
Sounds like Keith may have an issue with the playback still needing to be routed to his Focusrite, but that may be because his speakers are connected the Focusrite, or some other such setup/routing issue. Hard to say with the limited clues. Or maybe the 12 simply doesn't play nice in both directions.

A Control surface is a little different in that the sliders on the mixer are supposed to control the sliders in the DAW, as well as other stuff.
At least that's how I understand the basic definitions, if that helps.




I do not work here, but the benefits are still awesome
Make your sound your own!