Provisional assessment timelines: importers/exporters must furnish requested documents within prescribed periods; officers may grant limited extensions. Regulation 4 mandates that when a proper officer makes a provisional assessment due to missing documents or information, the officer must specify the required particulars in writing within fifteen days. The importer or exporter must supply those documents or information within two months, with a possible single two month extension by the proper officer and further discretionary extension by a supervising officer, provided reasons are recorded. Failure to furnish within the allowed period permits finalisation of the assessment on available record.