A subregion describes an area of the toposurface where a various set of properties can be used. As for instance, material.
Toposurface subregions belong to areas which should be drawn within current toposurfaces.
One can apply subregions to sketch parking lots on a graded surface, roads, or islands. To make the process easy apply a detailed component as a template and apply Pick Lines tool available in subregion editor. It is also possible to lock the boundaries of the subregion to the picked lines of the detail component. The subregion is modified automatically while shifting the detail component.
While generating subregions, no individual surfaces are produced. In order to make individual surfaces to be edited seperately, use the Split Surface tool.