The difference between the Preliminary and Mains stages of the UPSC exam lies in their purpose and format. The Preliminary exam is a screening test comprising two objective-type papers designed to eliminate candidates. Its focus is on general studies and aptitude. In contrast, the Mains examination includes nine descriptive papers that assess in-depth knowledge and analytical abilities. The Mains emphasizes the candidate's understanding of subjects, ethics, and essay writing. This structure ensures that only well-prepared candidates progress, allowing for a comprehensive evaluation of knowledge before the final interview round.