Was it decided by the government or did individuals have a say in what field they wanted to end up in? And did they fill the ranks of more specialized fields like doctors by those who did well in school or by volunteers?
I don't know english very well, but i'll try to describe this topic for you.
Firstly, after finishing school (10 years, starts 7 years old) typical soviet citizen had 3 options: find a job, go to military service (for men only, 1.5-3 years) or apply for college/university. First 2 options are obvious. Third one is more interesting.
If he passes exams well enough, he becomes freshman. After finishing college/university (5-6 years usually and 6+ for medical university) he usually has to complete the contract (not for all colleges/universities).
The contract is usually 2-3 years long, university administration gives 3-5 options where you he can go. For en example, let's imagine i finished a medical university and they give me 3 job offers from hospitals and i must choose one. Sometimes, some of the job offers can be from a hospital in another city. Also after accepting an offer i receive a flat (and becomes my ownership after 2 years of work) near my working place.
After completing the contract i can go where ever i want and find another job if i want, but usually people stayed at this job.