Most of the time, there is no difference. A "top" assembly, by definition is designed to fit into the pair of shelf supports in the top of a four post or case style shelving unit. A "regular" (R) assembly has been designed with pins which fit into the "keylock" holes in four post shelving uprights. Oblique recommends at least 8 shelf supports (4 pairs) should be used in every shelving unit. If a pair of supports is installed at the top of the unit and one in the bottom, then two more pairs should be distributed equally within the unit. We use these pairs with "supporting" assemblies which in most cases look exactly the same as "top" assemblies. However, since it is not physically installed at the top of the unit, we call it a "supporting" assembly to clarify its function.