How is the conversion rate set, and can it change?

Updated 

The conversion rate is set slightly higher than the current USDT-INR conversion rate in the spot market. The conversion rate is set slightly higher so that -

1. CoinDCX can hold the rate at a fixed price and absorb minor price changes.

2. This conversion rate remains same for both Buy and Sell allowing the users an opportunity to realize profits at a slightly higher price.