(a) A brief of an amicus curiae may be filed on consent of all parties, or by permission of the board granted on application to the board by the person seeking to file the brief, or at the request of the board. A copy of the proposed brief of the amicus curiae shall be filed conditionally with the application. The application shall specify the interest of the amicus curiae and state the reasons why a brief of an amicus curiae is useful in the appeal. Except by agreement of all parties, or by order of the board for good cause shown, the application for permission to file a brief of an amicus curiae shall be served and filed at the same period as the party whose conclusion the brief supports.

(b) A person who is not a party, nor desires to intervene or file an amicus curiae brief, may, in the discretion of the board, file a letter of no more than two pages concisely stating that person's interest in and position on specific issues on appeal. The person making such a limited appearance shall transmit a copy of the letter to each party, and the board's docket clerk will provide on request the addresses of such parties to a person making a limited appearance. Such letters shall be filed with the board's docket clerk prior to the filing of respondent's brief, or prior to oral argument if oral argument is requested, and after copies have been mailed to all parties by the person making the limited appearance.