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Answer is little subjective, Span is a system which calculates margin requirement at portfolio level to cover losses with 99.7 probability that can happen over 1 day timeframe. It consider various scenarios of changing price and volatility and conclude at “largest reasonable loss” that can happen over 1 day. So, this varies dynamically with changes in price and volatility through out the day.
Exposure is just a fixed set percentage on contract value, this is as a second layer check to keep losses under control in case span margin is breached, till now SEBI has given option on levying it but now considering the leverage, volatility etc it has made it compulsory to maintain it.
I personally believe if one has good trading system this leverage restriction is beneficial to them in long term as leverage is double edged sword and returns are not sustainable using higher leverage alone.