The term "usage charge" herein means the charge assessed by the Port Authority for facilities used by any carrier, shipper or consignee, stevedore, contractors, their agents or servants, and/or any other person when they perform car, barge or truck loading or unloading, or the use of said facilities for any other gainful purpose for which use a charge is not otherwise specified.
28 Miss. Code. R. § 1-01-60