Modify works with the surface by manipulating links and surface curves. The links of a surface characterize the shape of the surface. The collection of links in one direction across the surface is known as a surface curve. When you select the surface to modify, a number of blue lines appear with intersection points. These are the links and surface curves. A link is any segment between two intersection points.
Isolines are displayed on the screen, but they are not the surface curves that are edited. To see the isolines while editing, select the Show surface option. With Modify you can change specific points in the surface to fine tune your surfaces to match your needs. Changing the surface with Modify breaks the parametric link of the surface to the original construction data.