Seniority in Civil Services is accounted cadre/service wise and its policy is governed under Section 8 of Civil Servants Act, 1973; and Civil Servants Seniority Rules 1993 which were formulated in light of Section 8 of Civil Servants Act 1973.
One can find Civil Servants Seniority Rules, 1993 from this link
http://www.scribd.com/doc/25334727/P...Seniority-Rule