By using a plane, you can see how Maya treats any surface as if it is a volume when performing Booleans. Start a new scene. Go into component mode and pull the middle two rows of CVs up to add curvature to the surface. Duplicate both the sphere and the plane and move them to the side. Be sure that you are in shaded mode.
Lines protrude out from the surface to indicate the surface's normal direction.
Now this surface's normals are going in the opposite direction. Click on one of the spheres then press the Enter key to select it as the first object. Click on the corresponding plane to select it as the second object. The shapes are subtracted and the sphere now has the cylinder removed from its surface. You will notice that in the results are the opposite.