File>New
Scale Cube by x = 2
Add Mesh>Cube (makes Cube.001)
Scale Cube.001 by y = 2
Select Cube, Add Boolean modifier Union
(you find this situation in the attached file).
Now apply the modifier. You see 2x4 "flat" edges in the cross-shaped sides of the result mesh.
Now try to remove those by executing the "limited dissolve" operation.
Result was that there were 4 edges left than cannot be selected.
If, from Cube, you select All and delete all vertices, edges & faces, 12 "edges" remain, that are visually identical to the vertices of Cube.001{F82301}