This is a very confusing image to view, even for me and I took the photo. It’s hard to tell exactly what you’re looking at. It is a mezzanine-like area shot from across a space that is open down to the main level in Maison La Roche. The lower half of the image is the short railing wall overlooking the lower level. The vertical line is the end of a black (or dark) divider wall separating a room on the left from an walkway along the wall on the right.
I’m not sure if it was before or after the fact that I realized this composition is reminiscent of one of my favorite photos by the Hungarian photographer