(a) Subject to this section, the following persons, associations, and entities shall be admitted as parties to a contested case hearing under Section 2703.202, the periodic hearing under Section 2703.203, or a hearing under Section 2703.206:
(1) a trade association whose membership is composed of at least 20 percent of the members of an industry or group represented by the trade association;
(2) an association, person, or entity described by Section 2703.202(b);
(3) an interested person described by Section 2703.202(b-1); or
(4) department staff.
(b) A party to any portion of the periodic hearing relating to ratemaking may request that the commissioner remove any other party to that portion of the hearing on the grounds that the other party does not have a substantial interest in the subject matter of the hearing. A decision of the commission to deny or grant the request is final and subject to appeal in accordance with Section 36.202.