In a picture, unbounded areas are interrupted by the picture edge, so their unboundedness is an illusion produced by the artist’s skill in organizing the closed forms within the picture (The Grid and the Rectangle). One has to suggest an enveloping medium, whether it has depth or not. Emptiness doesn’t always work that way. As in many of Cézanne’s watercolors, the empty spaces might contain parts that we know are there, but haven’t been explicitly rendered.
In later model modernist abstractions, unpainted bits just fit into the composition and don’t take us anywhere beyond that. In these two cases, the empty parts are not unbounded. Unboundedness is a perspective on the rest of the picture, and, as such, it is a formal equivalent of the installation strategy that brings the surrounding social world to bear on the artwork.