220 ILCS 5/9-102
Every public utility shall file with the Commission and shall print and keep open to public inspection schedules showing all rates and other charges, and classifications, which are in force at the time for any product or commodity furnished or to be furnished by it, or for any service performed by it, or for any service in connection therewith, or performed by any public utility controlled or operated by it. Every public utility shall file with and as a part of such schedule and shall state separately all rules, regulations, storage or other charges, privileges and contracts that in any manner affect the rates charged or to be charged for any service. Such schedule shall be filed for all services performed wholly or partly within this State, and the rates and other charges and classifications shall not, without the consent of the Commission, exceed those in effect on December 31, 1985. But nothing in this section shall prevent the Commission from approving or fixing rates or other charges or classifications from time to time, in excess of or less than those shown by said schedules.
Where a schedule of joint rates or other charges, or classifications is or may be in force between two or more public utilities such schedules shall in like manner be printed and filed with the Commission, and so much thereof as the Commission shall deem necessary for the use of the public shall be filed in every office of such public utility in accordance with the terms of Section 9-103 of this Act. Unless otherwise ordered by the Commission a schedule showing such joint rates or other charges, or classifications need not be filed with the Commission by more than one of the parties to it: Provided, that there is also filed with the Commission a concurrence in such schedule by each of the other parties thereto.
Every public utility shall file with the Commission copies of all contracts, agreements or arrangements with other public utilities, in relation to any service, product or commodity affected by the provisions of this Act, to which it may be a party, and copies of all other contracts, agreements or arrangements with any other person or corporation affecting in the judgment of the Commission the cost to such public utility of any service, product or commodity.
220 ILCS 5/9-102