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CSS
Classification of the Service
CSS officer fall under two groups under Central Civil Service i.e. Group A and Group B. 

1. Selection Grade (Deputy Secretary or equivalent) -Belongs to Central Civil Service Group ‘A’
2. Grade-I (Under Secretary or equivalent) -Belongs to Central Civil Service Group ‘A’
3. Section Officer Grade -Belongs to Central Civil Service Group-‘B’ (Ministerial)
4. Assistant Grade -Central Civil Service Group ‘B’ (Ministerial)           

Group A officer i.e. those who belong to Grade I i.e Under Secretary and then the Selection Grade of Deputy Secretary and Directors are direct recruits from the Combined Civil Service Exam that is held all over India. Someone who belongs to Section officer and Assistant Grade are non recruits from the Civil Service Exam but are recruited through the SSC. Only a direct recruit through civil service exam can become or allotted to become an officer in the CSS cadre of posts of Under Secretary, Deputy Secretary and then Senior selection grade of Director then SAG level of Joint and Additional Secretaries.

CSS class
CSS officer total of this date to 10,000 candidates to which 2000 only belong to the Group A who are recruits through the civil service exam whereas the rest 8000 belong to the Group B and are non recruits through the non civil service exam.
See Group A Civil List (http://persmin.nic.in/CSWing/CSDivision/CSS/css_civillist/civil_list_css_2010.pdf) - See page 5 including the cover page written by the Minister.

Posts across and hold across Central Government
 See Group A Civil List (http://persmin.nic.in/CSWing/CSDivision/CSS/css_civillist/civil_list_css_2010.pdf)
If you see the civil list of CSS cadre officer of Grade A then you will see candidates are holding position all the way from Under Secretary to Deputy Secretary (both junior and senior) to Directors to Joint Secretary and even to few Additional Secretaries. 

This means that CSS group A cadre officers are holding positions from low to mid to high level executive posts in the Central Government. It is just wrong to say that they hold only lower and middle level positions in the Government. As per history, the CSS was devised primarily to manage the middle level posts in the Central Secretariat, which is the Headquarters of the Union Government but if you see the Civil list it can be clearly seen that since independence CSS has evolved and grown. Hence the Group A officer also holds posts as Joint and Additional Secretary position which is not a middle level but a SAG level position.

Group A and Group B
CSS officers belong to group A who are direct recruits and the ones who belong to Group B who are recruits through non civil service exam. You should know that both Group A and Group B officer belong to both Central Civil Services of the Government of India.
 See Group A classification  (http://www.persmin.nic.in/EmployeesCorner/Acts_Rules/ccs(cca)/SCHEDULE-1.PDF)
 See Group B classification  (http://www.persmin.nic.in/EmployeesCorner/Acts_Rules/ccs(cca)/SCHEDULE-2.PDF)  — Preceding unsigned comment added by 59.178.160.221   "