Jurisdiction of tax commissioners: Board directs areas, persons or incomes and specifies allocation and concurrent duties. Section 121 vests in the Board the power to direct the areas, persons or classes of persons, incomes or classes of income, and cases or classes of cases in respect of which Commissioners shall perform their functions. Earlier language provided for concurrent jurisdiction where two or more Commissioners were assigned the same scope, subject to Board orders for distribution and allocation of work; amendments subsequently revised and later omitted that specific paragraph, emphasizing the Board's role in specification and allocation.