A surface's orientation may be specified by choosing one of the two oppositely oriented unit normal vector fields along the surface (Revit uses only orientable surfaces). A surface's parametric orientation can be defined by the vector field defined as the unitized cross product of the surface's first partial derivatives: dS/du x dS/dv. A surface's orientation is then defined by getOrientationMatchesParametricOrientation(); if getOrientationMatchesParametricOrientation() is true, the surface's orientation is the same as its parametric orientation, otherwise it is opposite to its parametric orientation. See also Face::OrientationMatchesSurfaceOrientation.