See in the starting years of the career of an IFS, they are posted in the embassy's of different countries. Usually these postings are of 3-5 years tenure and after this tenure the officer is posted again to any other country. For the first 15 years of the service, IFS officers are posted in embassies. After that based on the performance, many officers becomes bureaucrat in Ministry of Foreign Affairs or in PM Office. So it depends on performance of a officer but in general as the officer gains experience they are usually asked to do job in administrative sector of international relations.

TO know more about career as an IFS visit the link below:

https://www.careers360.com/careers/indian-foreign-service-officer

To know more about UPSC CSE visit the link below:

https://competition.careers360.com/exams/upsc-civil-services-exam

I hope this answer helps. All the very best for your future endeavors!