HifiBerry Digi+ Pro ADC

Volumio Information

Volumio Version:2.779
Hardware: RPI 3B+ & HifiBerry Digi+ Pro

I just bought a RPI 3B+ with the HifiBerry Digi+ Pro HAT and installed Volumio. It works like a charm but I have a question regarding ADC. For playback I have set the following:
Output device: HifiBerry Digi+ Pro
DAC model: HifiBerry Digi+ Pro

And in the menu I now see this:
I can select or deselect this, what does this do exactly?

This available input of the Dac. You can plug an external source (I don’t know how though, maybe a sprig or coax connector?)

I did not know there was input possible on the Digi+ Pro. I’ll just leave it disabled. Thnx

Same question here. I don’t think the DIgi+ Pro has an input for external sources, only the Digi+ I/O has.