law: 51. Power to levy concessional rates in certain cases.-
  In framing scales under any of the foregoing provisions of this Chapter, the 4[Authority] may prescribe a lower rate in respect of,-
  (a) costal goods, that is to say, goods, other than imported goods as defined in the Customs Act, 1962 (52 of 1962), carried in a vessel from one Indian port to another Indian port:
  Provided that the 4[Authority] shall not make any discrimination between one Indian port and another such port in prescribing a lower rate under this section;
  (b) other goods, in special cases.