What is an illuminated source?
An illuminated source is something that is assisting in the viewing of something. It will allow people to see it when they normally cannot see it.
The penumbra is the part of the shadow from which PART but not all of the light source is visible, so that region is partly but not fully illuminated. The umbra is the part of the shadow from which NONE of the light source is visible, so that region is not illuminated at all, not even a little. That's why.