The CASA reg formerly required an aircraft to be free of snow/frost/ice. Most jet operators will describe the formation of a very thin layer of ice on the underside of wings where the skin meets the fuel tanks. The cold fuel after a long cruise causes this layer of frost to form as you descend into more humid air. As correctly described above, most AFM/FCOMs permit a small layer of ice in the vicinity of the fuel tanks, to allow dispatch before the ice fully melts. The CASA change allows a more sensible approach and removes the requirement for a deice to legally dispatch the jet in the event of short turns.