Assessment of non-filers: officers may make best-judgment assessments; filing a return can withdraw the order but interest remains. Where a registered person fails to furnish the required return after notice, the proper officer may assess tax to the best of his judgment and issue an assessment order within five years from the date specified for filing the annual return for the relevant financial year. If a valid return is furnished within thirty days of service of that assessment order, the order is deemed withdrawn, but interest and late fee liabilities continue.