Provisional customs assessment governs duty adjustment, interest, and refund obligations when final assessment is delayed or revised. Provisionally assessed customs duty may be made where self-assessment is not possible, testing or further enquiry is needed, or documents or information remain pending, subject to security for any shortfall. The proper officer must finalise the assessment within the prescribed time, with limited extension and special exclusion periods where finalisation is delayed by foreign legal process, pending judicial proceedings, stays, Board directions, or settlement-related applications. On final assessment or re-assessment, amounts are adjusted, deficiencies are payable, excess is refundable, interest applies on unpaid duty and delayed refunds, and refunds are paid only where the incidence has not been passed on in specified cases.