Yes, after STP, they are posted out of province. Preferences are taken into account but depends on your merit also.
Girls are posted in their province.
After 3 and half years, you have the right to get posted in your province but not necessarily true for every1 as people serve upto 5/6 years outside province also. Even after returning to home province, one might be posted out of province again as its a Federal service.
In PAS, postings and your stay in home province/ at a station depends alot on your links, political connections and family background. Without it, its tough journey in PAS.