Windows Sound mixer options - Speakers vs. Digital Output

[chrisr34000]

Hello,

I have an Asus Xonar D2X which is connected via SPDIF out to the coax input of an AMP/DAC.

The windows sound mixers offers me following output options:

 

 

I get sound playback over "Digital Output" as well as over "Speakers", however the sound is different. Over "Digital Output" I experience a much lower sound volume. Now I'm confused why I have two playback options and why they're so different. Do you know what's the difference between the two?

Also: What is "Asus Xonar D2X Converter"?