Tool Palettes Surfaces > Planar surfaces > Set planar > To create a planar surface

Surfaces > Planar surfaces > Set planar

Creates a trimmed NURBS surface from a set of planar boundary curves.

See Create a flat surface inside a curve

To create a planar surface

Pick individual curves, or draw a pick box around all curves to be made into planar surfaces.
Choose Surfaces > Planar surfaces > Set planar.
Check that all curves are picked, and click Go.

The curves are turned into flat trimmed surfaces.

Create History

Save the history of the new planar surface for later editing. If you turn Create History on, you can modify the curves that were used to create the surface, and the surface will update.