Exchange of information enables tax authorities to obtain and use relevant taxpayer data for assessment and enforcement purposes. Article 27 creates a reciprocal obligation for competent authorities to exchange documents and certified copies foreseeably relevant to implementing the Agreement and administering or enforcing domestic tax laws, requiring the requested State to use its information gathering measures even without domestic need, subject to limitations. Received information must be treated as secret and disclosed only to persons or authorities involved in assessment, collection, enforcement, prosecution, appeals or oversight and used only for those purposes unless both States' laws allow other uses and the supplying authority authorizes them. Exceptions prohibit measures contrary to domestic law or practice, supplying unobtainable information, or disclosing trade secrets or information contrary to public policy, but do not allow refusal solely because information is held by financial intermediaries or concerns ownership interests.