It means that if the FAA has prescribed any routes or altitudes, then the operator must comply with them. If not, the operation may go below 500 ft as long as the operation does not pose hazards to persons or property on the surface.
Previous wording was less, I mean more ambiguous:
"(d) Helicopters. Helicopters may be operated at less than the minimums prescribed in paragraph (b) or (c) of this section if the operation is conducted without hazard to persons or property on the surface. In addition, each person operating a helicopter shall comply with any routes or altitudes specifically prescribed for helicopters by the Administrator."