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 being made for purposes of representation or assistance. Where a person is to act as a representative of a party, the arbitral tribunal, on its own initiative or at the request of any party, may at any time require proof of authority granted to the representative in such a form as the arbitral tribunal may determine.