Where by this or the special Act the Commissioners are authorized to order that any Rate shall be levied by Assessments to be made for separate and distinct Districts, the Commissioners from Time to Time may order Assessments to be made in respect of the Rates authorized to be so levied upon separate and distinct Districts, and in such Case the Commissioners shall cause their Surveyor to describe and define in the Plan of the Town or District within the Limits of the special Act every such separate and distinct District for the Purposes of separate rating as aforesaid, and so from Time to Time as Occasion shall require.