The backing surface for this ISurfacePatch.
Used correctly, this backing surface is a strict geometric superset of the space defined by ISurfacePatch itself. In other words, for every spatial query against the ISurfacePatch for which a solution exists (a raycast that hits the surface, for example), that same solution should be a valid solution to the same query for the BackingSurface; however, there may exist spatial queries for which the ISurfacePatch has no solution, but a solution exists for the BackingSurface.