The CPlane command sets the origin and orientation of the construction plane in the active viewport. Pick a new origin point or choose an option. The construction planes in all viewports change so their origin is at the picked location. The Curve option sets the construction plane perpendicular to a curve.
How do you reset a CPlane?
To reset the CPlane, use CPlane (World) and select the worldview you would like the CPlane to lie in.