Assessment of unregistered persons permits best judgment tax assessment within a five year period, subject to opportunity to be heard. Section 63 authorises the proper officer to proceed to a best judgment assessment of a taxable person who failed to obtain registration or whose registration was cancelled but who remained liable, for the relevant tax periods, and to issue an assessment order within five years from the date specified under section 44 for furnishing the annual return for the relevant financial year, provided the person is given an opportunity of being heard.