The criterion for selecting candidates for the final merit list in UPSC encompasses performance in all stages of the examination, which includes the Preliminary examination, Main examination, and the Personality Test. Initially, candidates must qualify in the preliminary exam to advance to the mains. Subsequently, their performance in the mains, assessed through written papers, contributes to the merit list. Finally, the personality test adds a human element, evaluating candidates' suitability for civil services. This comprehensive evaluation process helps ensure that the selected candidates are both knowledgeable and competent.