SPAN margin is the minimum prescribed margin that’s blocked for F&O writing positions based on the exchange’s instruction.
The exposure margin is a type of margin that’s blocked over and above the SPAN margin to mitigate any MTM losses.
SPAN and exposure margins are determined by the exchange. So when you initiate a futures trade, you have to adhere to the initial margin requirement. The total initial margin, i.e., SPAN + exposure, is blocked by the exchange.