FS Global is a set of global mesh, but mesh in itself is merely the description of orography, it may be more or less detailed, but is nothing more, while the various UTX packages add accurate roads, railroads, coastlines, landclasses and sometimes more detailed land polygons that add themselves to the underlying mesh to create a better scenery.

You can say that the various UTX is a more accurate layer to put over mesh and the default terrain landclass to give a more correct representation of the real world into the virtual one... using obviously an add-on, seen that them at M$ can't be bothered to do it themselves.