It can’t dissolve into itself. If you select a single face and dissolve the face you still just have that face, because there’s nothing to dissolve. you have to have 2 or more face connected to be able to dissolve a face. Same is true for an edge. If the edge is only connected to 1 face then it can’t dissolve. Vertices are a little different in that they need two edges connected to them to be able to dissolve.
