Assessment of unregistered persons allows tax authorities to assess liable but unregistered taxpayers to best judgment after a hearing. Where a person liable to register fails to obtain registration or has had registration cancelled but remained liable to pay tax, the proper officer may assess the person's tax liability to the best of his judgment for the relevant tax periods, issue an assessment order within five years from the date specified for furnishing the annual return for the financial year to which the unpaid tax relates, and must give the person an opportunity of being heard.