There seems to be two “grounds” of being, one that is associated with awakening as the Witness and one that is revealed when awakening as the nondual.
We find the Witness ground when we find ourselves as formless awareness, that in which our human self and the world of form arises within. It is not really located anywhere, and it allows anything and everything to come and go as guests – as they do anyway.
But still, in my own experience, there is a sense of a veil here, a slight dullness. It is not truly a ground of all there is. It is a more “personal” ground, associated with an identification as the Witness, as the seer.
When we awaken to the nondual, there is just what is happening – as it is – and there is no “I” anywhere.
In my experience, the subtle veil is lifted here. The ground is revealed as crystal clear, with a diamond quality. The slight dullness that came with the identification as the Witness is lifted. It is similar to going from low-land air (still pretty clean) to the most crystal clear mountain air.
Space & Clarity
Again from my own experience, it seems that the most distinct characteristic of the Witness ground is spaciousness – infinity, while that of the nondual ground is its crystal clarity – its diamond quality.