The weightage of the optional subject in the UPSC Mains exam is significant, accounting for a substantial portion of the total marks. Candidates choose one optional subject that consists of two papers, each carrying 250 marks, totaling 500 marks out of the overall 1750 marks for the Mains. This gives the optional subject a decisive impact on a candidate's overall performance and ranking. Choosing a suitable and well-prepared optional subject is crucial, as it allows candidates to leverage their strengths and knowledge, helping them to maximize their scores effectively.