The top mounted are installed on the ceiling and flush water directly downwards.  In any case, the sprinklers of the shower heads may be adjustable. Single mounts are those that are installed as a single head with a regulator or valve. This head may have a low or high flow rate, depending on the rating that it is constructed with.  It may also be designed with some spray holes that will ensure not only the flow rate but also the pulsation of the water.