Creates a planar (flat) surface within a boundary curve.
Complete creates the planar surface along the entire curve.
Partial lets you display manipulators on the planar surface with the Show Manipulator Tool and edit the resulting planar surface along part of the input curve. Click the input curve or isoparms’s heading in the Channel Box to display the options.
Creating trimmed surfaces from open curves
You do not necessarily need a single boundary curve to create a trimmed surface, however, you do need an enclosed region. This can be created by intersecting several curves. You can create trimmed surfaces using curves with overlapping boundaries. Select the curves and use Surfaces > Planar to create the trimmed surfaces