Exchange of information: mutual tax information sharing permitted for enforcement, subject to secrecy and defined exceptions. Article 27 mandates exchange of information, including documents, necessary for applying the DTAA and related domestic tax laws, subject to secrecy: received information may be disclosed only to persons or authorities involved in assessment, collection, enforcement, prosecution, or appeals and used solely for those purposes, with permitted disclosure in public court proceedings. The Article exempts any obligation to take measures contrary to a State's laws or practices, to supply information not obtainable under law or normal administration, or to disclose trade or professional secrets or information contrary to public policy.