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        From the Operations section of the action bar,
            click
          Top and Bottom Faces
           .
        In the Top and Bottom Faces panel, 
        
          
            Under the Design Space References area, select the space
              references.
            
              
            Under the Floor Surfaces area, select the surfaces.
            
              
        Click Apply.
        
          
            The surfaces are set as top and bottom surfaces to the corresponding space
              references, and are listed in the Top Surfaces and
                Bottom Surfaces columns.The top and bottom surfaces are saved as attributes in the space reference and are
              used to recognize the top and bottom faces of the space reference.
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