Best judgment assessment permits the assessing officer to determine chargeable expenditure when returns or accounts are missing or incomplete. Section 10 provides that where a person fails to file a required return, fails to comply with a notice, or the Assessing Officer is not satisfied with the correctness or completeness of accounts, the Assessing Officer shall, after taking into account all relevant material gathered, make a written best judgment assessment of the chargeable expenditure and determine the sum payable or refundable on that basis.