The appellants take the position that the surface portion of their lands in the City of Toronto, which were used for surface parking and the placement of advertising signs, were to be properly classified for purposes of the Assessment Act (Ontario) in the Commercial Property Class and that the “air rights” above the parking lot surfaces were properly classified in the Commercial Property Class, Vacant Land Subclass – whereas the position of the Municipal Property Assessment Corporation (“MPAC”) was that the density potential or development potential defined by the City zoning by-laws