The  Shaded function sets the render mode to shadeded. The render mode displays 3D bodies (solids, surfaces, and shells) with a rough shaded representation. When a model is rendered with the  Shaded option, each facet is displayed with a constant color, whereas in smooth shaded modes (Gouraud and Phong) the colors blur across each facet, causing them to disappear. In this mode bodies will obscure any entity or portion of entity that is behind it when the model is viewed from a particular orientation. Flat is one of the three shaded render modes (Flat, Gouraud, Phong) which use color shading of the faces of the body. This shading can be used in the Format right context menu, (right click solid and choose Format>Rendering).        

This mode may be useful as a diagnostic tool to be able to see the facets on a body.

Above > An example of a solid cylinder, displayed using the Flat Shaded feature