If the geometry of a surface, for which you must remove some of the existing boundary lines, changes subsequently, you do not need to redefine the surface. By double-clicking the relevant surface, you open the "Edit Surface" dialog box. You can use the Select function to select the boundary lines. If lines that are already in the list are selected again, they are removed from the list. This option allows you to save a lot of time, especially for geometrically complex surfaces with many boundary lines.