Create a flat surface inside a curve
Use the Planar Surfaces > Set Planar tool 
in the Surfaces tool palette to create a trimmed surface using a flat curve or set of curves as the outline. The curve or set of curves must define closed regions.
Fill a flat curve with a planar trimmed surface
In the Surfaces tool palette, select the Planar Surfaces > Set Planar tool
.Pick individual curves, or drag a pick box around all the curves that define the planar surface or surfaces.
Check that all curves are picked, and click Go.
A flat trimmed surface is created inside the curves.
How does the layout of multiple curves affect the surface?
Curves that cross other curves are added together.
Curves that are completely enclosed by other curves become holes.
Curves may be separate from each other. This will create the appearance of separate surfaces.
You can mix boundary curves of different degrees without affecting the resulting surface.
The flatness of surfaces created with the Set Planar tool is not automatically maintained.