RBI’s income comes mainly through interest on the securities it holds and in 2017-18 the largest component of expenditure was a provision of about Rs 14,200 crore it made to the contingency fund. RBI's surplus represents the amount it transfers to the government. There are two unique features about RBI’s financial statements.

from Times of India https://timesofindia.indiatimes.com/business/india-business/what-are-rbis-surplus-funds-where-do-reserves-come-from/articleshow/70849771.cms