So I went across one of the few games the Nvidia's Ansel feature supports, and within the many types of capture type it provides, aside from:
"screenshot", "super resolution" and "360º"
"stereo" and "360º stereo".
What "360º stereo" do is take a screenshot and duplicate on top of it, resulting in two rows of the same screenshot being displayed in the same image. As for "stereo" is the same thing, but as two columns.
Note that if you upload a 360 screenshot on facebook, it will automatically become an interactive viewport to scroll around. But for STEREO 360s I can't find anything useful, not even when basically opening it from a casual photo viewer (such as Windows' integrated photo viewer). So what are the so-called "stereo screenshots" (or images) really for?