Planes are used in some Surface Profile procedures (Contour Milling, Profile: Open by Plane, Closed by Plane, and Contour Milling: CurveMx 3x) instead of using part surfaces.
Planes are created using the Environment / Plane function. You can create planes in the Part application by switching to the CAD environment.
Press the 
icon in the
Procedure Guide, and
you will see the Geometry Parameter
Table.
Press the box next to Plane, and you are ready to pick your plane. The cursor has the following appearance when picking a plane:
