220 ILCS 5/10-112
Every order of the Commission shall be served upon every person or corporation to be affected thereby by personal delivery of a copy thereof, by mailing in the United States mail a copy thereof in a sealed package with postage prepaid, or by electronic means to the person to be affected thereby or in the case of a corporation, to any officer or agent thereof upon whom a summons of a circuit court may be served in a civil action. Where such persons or corporations, or both, exceed 3 in number, service as herein provided may be upon the attorneys or representatives of record, if there be any; and in any event, mailing in the United States mail as herein provided, shall constitute service, without additional proof of a receipt of such copy or copies of such order. Within a time specified in the order of the Commission every person and corporation upon whom it is served must, if so required in the order, notify the Commission in like manner whether the terms of the order are accepted and will be obeyed.
220 ILCS 5/10-112