The second paper of IAS prelimnary Examination of UPSC is popularly called CSAT or Civil Services Aptitude Test. It is of qualifying naturein IAS syllabus. The CSAT was introduced to end the use of scaling system for varying subjects in the General Studies paper. CSAT was introduced in 2011 to IAS syllabus.
The benefit of introducing aptitude tests in the IAS syllabus is that it assesses your logical reasoning and thinking performance. It is based on multiple choice questions answered in exam conditions. No subject influences, or can have an effect on an aptitude test.