The board shall assign a case number to each petition for review which shall be the official reference number for purposes of identification. The first two digits of the case number shall correspond to the last two digits of the calendar year in which the petition was filed. The third digit shall designate which regional panel has jurisdiction over the matter. The Eastern Washington panel shall use the digit "1"; the Western Washington panel shall use the digit "2"; and the Central Puget Sound panel shall use the digit "3." The last four digits shall be numbered sequentially in order of receipt. Consolidated cases are generally assigned the number of the last filed petition followed by a "c."