This entity defines an action which is distributed over a surface. A surface action can be connected with a surface member or surface connection. 

Coordinate systems and related behavior is the same as for IfcStructuralActivity. 

• Standard Case: If connected with a surface item and acting on its entirety, instances of IfcStructuralCurveAction have no an ObjectPlacement nor a Representation. It is implied that the placement and representation of the IfcStructuralActivity is the same as the ones of the member or connection.
• Special Case 1: If connected with a surface item but acting only on a part of it, instances of IfcStructuralSurfaceAction have an ObjectPlacement and Representation, containing an IfcFaceSurface which topologically defines the loaded part of the surface. See IfcStructuralActivity for further definitions.
• Special Case 2: If not connected with a structural item (which can happen in an incomplete or conceptual model), a surface action should have an ObjectPlacement and Representation, containing an IfcFaceSurface. See IfcStructuralActivity for further definitions.