Flat surfaces are not based on
splines. Flat surface is the rectangular mesh-surface, that can be used
to produce other objects and complex meshes. To create Flat surfaces
choose Create.../Surfaces.../Flat method and draw the rectangle
by holding the left mouse button:
A dialog box will apper where you
should:
1) Enter the name for the new object
2) Enter the number of horizontal and vertical steps:
As you see on the result image
the surface has two horizontal and three vertical steps in.