Self-assessment of customs duty required, with verification, re-assessment, speaking orders and audit powers for officers. Importers and exporters must self-assess customs duty on goods entered under the relevant entry provisions; proper officers may verify assessments by examining or testing goods and demanding production of contracts, broker's notes, insurance policies, catalogues or other documents and information. If verification reveals incorrect self-assessment on valuation, classification, exemption or concession, the proper officer may re-assess duty. Where re-assessment contradicts the declarant and is not accepted in writing, the officer must pass a speaking order within the prescribed short period; failing re-assessment or speaking order, the officer may audit assessments at the office or at the declarant's premises.