The process of finalizing cadre allocation for successful UPSC candidates is based on their rankings, preferences, and reservation policies. After the examination results are announced, candidates submit their service preferences, which are then matched with their ranks and the available vacancies in various services. The Government of India holds the authority to allocate services based on merit and policy considerations. The allocation not only reflects the candidate's performance but also aims to meet administrative needs, considering factors like regional representation and current service demand.