This question was asked from a senior Bureaucrat of 2nd common (PSP)

His Answer States That: The seniority in service is determined by the CSS marks + Performance at CSA + Performance during STP, then you get the final seniority list which means the real competition of a probationer is with his/her Group/Service mates. As far as the persons are concerned who have been in Govt job previously, the only benefit what they get is that their pay is started with previous increments. No other benefits, previous Govt job experience is nothing to do with one's seniority in list of his/her batch fellows.

I think the above answer is quite explanatory..!!!!