never hear about ALSA -- but probably because this is only for Linux right?
ALSA is Linux specific and provides driver support. Another important point I forgot to add was that Linux also natively supports USB Audio Class 1 & 2 compliant devices (mostly).
Out of curiosity it just refers to sound "cards" there are a lot of external DACS not listed .... do you mean that with the ID a Naim, Schitt, or Pacific Audio DAC probably won't work or am I really lost ?
All USB Audio Class Compliant devices (class 1 or 2)
should work, but some folks have had issues with certain XMOS models. If the DAC is not UAC 1 or 2 compliant and is not listed in the ALSA matrix, it almost certainly won't work. I think the Schitt devices are UAC compliant, so they'll be fine. I know nothing about NAIM or Pac Audio.
However, I can confirm that, for example, my Steinberg doesn't work with Linux (which is not surprising because it's not UAC class compliant, and it's not listed on the ALSA matrix).
Welcome to the world of open source, where drivers are written for free by hobbyists and the best way to find out if something works is by trying it