Each party may be represented or assisted by persons chosen by it. The names and addresses of such persons must be communicated to all parties and to the arbitral tribunal. Such communication must specify whether the appointment is b eing made for purposes of r epresentation or assistance. Where a person is to act as a representative of a party, the arbitral tri- bunal, on its own i nitiative or at the request of any party, may at any time require proof of a uthority granted to the r epresentative in such a form as the arbitral tribunal may determine.