Since the lines do not necessarily have to rest in one plane, you can use this surface type to model curved surfaces, such as shells or membranes.
Complex geometries can be modeled by a suitable distribution into surfaces with four or three lines. RFEM generates the corresponding quadrangle surfaces from the geometrical conditions of the boundary lines.
When generating an FE mesh, curved surfaces are utilized by planar elements. The inclination angle of the finite elements from the plane can be set in the FE mesh settings in the "FE mesh" dialog box.