(A) Any person may petition for leave to intervene at any stage of a proceeding before the commission. A petitioner for intervention shall have the burden of showing an interest which is or may be affected by the outcome of a proceeding before the commission.
(B) The commission may consider the following in determining whether intervention is appropriate:
(1) The nature of the issues;
(2) The adequacy of representation of the petitioner's interest which is provided by the existing parties to the proceeding; and
(3) The ability of the petitioner to present relevant evidence and argument.
(C) Any person granted leave to intervene in a proceeding may participate in such proceeding as a full party, or, if desired by the Intervenor or so determined by the commission, in a capacity less than that of a full party. If an Intervenor is to participate in a limited capacity, the extent and the terms of the participation shall be in the discretion of the commission and shall be set forth when the commission rules upon the petition for intervention.
(D) The commission may allow the filing of briefs and permit oral argument at hearing by an amicus curiae. A person wishing to participate in an appeal as an amicus curiae shall move the commission for permission to so participate. Amicus curiae participation is discretionary and will be permitted only upon the terms and conditions imposed by the commission.