(a) Parties to a proceeding set forth either at §747.1 of subpart A or in subpart C, E or G of this part may obtain discovery only through the production of documents. No other form of discovery shall be allowed.

(b) In the event that a person producing documents pursuant to a document subpoena is permitted to be deposed, all questioning shall be strictly limited to the identification of documents produced by that person and a reasonable examination to determine whether the subpoenaed person made an adequate search for, and has produced, all subpoenaed documents.