What was the decision of the Apex Court with respect of civil services in Mohan Kumar Singhania v. Union of India?

In Mohan Kumar Singhania v. Union of India, the Supreme Court has ruled that each of the various civil services, namely, I.A.S.,I.F.S.,I.P.S., Group A Services and Group B Services, is a separate and determinate service forming a distinct cadre and that each of the Services is founded on intelligible differentia which on rational grounds distinguishes persons grouped together from those left out and that the differences are “real and substantial” having a “rational and reasonable nexus” to the “objects sought to be achieved”.