I recently got a picture in which quota was listed for CSS for each province with no of seats available per department. I saw some areas like GB, AJK didn't have any seat in PAS, PSP for that respective year.

1- I want to ask is there any seats on merit also? like some seats are fixed for merit disregarding domicile of a selected candidate?

2- If merit seats are available then how to get them? I mean a person applying from Punjab domicile can also apply for merit seat?

3- Is there anything like top 20, 30 or 50 candidates will get seat in their choice of department, without any need for specific province domicile?

From FPSC's website:
"Question No. 37

What is percentage of Regional/Provincial quota for seats/posts filled up by CSS Exam?

Answer:

Detail of regional quota is as under:
Merit=7.5%, Punjab=50%, Sindh=19% (Sindh(U)-7.6%, Sindh(R)-11.4%), Khyber Pukhtoonkhwa=11.5%, Balochistan=6%, GB/FATA=4%, AJK=2%.

Note:

10% quota is reserved for women from the share of Provinces/Regions, and un-filled vacancies will be carried forward to next examination..

5% quota is reserved for Minorities (Non-Muslims) from the share of Provinces/Regions. Unfilled vacancies will be carried forward to next examination."

You dont SELECT/APPLY for merit quota. Your score in the CSS exams does the talking for you. The higher the score/position, the higher your chance of being in the merit quota.

For Punjab, the Big Three occupational group's merit quota ends at 10-15 position, while the Punjab Quota ends till 30-40 position. Coveted groups like PSP or DMG sometimes are thus not available for Punjab Domicile, even when you score the 50th position. However, due to relatively low competition and lower number of successful applicants a 130th position from Balochistan can get allocated in DMG group, while a 71st position Punjab domiciled candidate can only get allocated in IRS group. In the latter's case, his competition was with other Punjab candidates. So all Big Three's seats were filled before his turn came around.

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great explanation, thanks a lot