Section 40(2)(ii) in The Bihar Finance Service Rules, 1953
(ii)The selection shall be made from amongst Commercial Taxes Officers who have completed not less than four years of continuous service on the first day of January of the year in which the promotion is made. The above period of service shall be reduced by one year in the case of officers belonging to Scheduled Castes and Scheduled Tribes, if adequate number of officers of those castes with prescribed minimum length of service are not available in the required proportion for the purposes of sub-rule (1).