The significance of the CSAT paper in the UPSC preliminary exam lies in its role in determining a candidate's aptitude for civil services. It assesses basic comprehension, logical reasoning, analytical ability, and numerical skills, all of which are essential for effective administration. Although qualifying in CSAT is mandatory, its marks are not counted towards the final merit list. This sets a foundational standard that candidates must meet to progress to the Mains stage. A solid performance in CSAT signals that the candidate possesses the requisite skills for problem-solving and critical thinking required in civil services.