Initial margin is defined as a percentage of your open position and is set for different positions by the exchange or clearing house. The factors that decide the amount of initial margin are the average volatility of the stock in concern over a specified period of time and the interest cost. Initial margin amounts fluctuate daily depending on the market value of your open positions.
Mark-to-Market margin covers the difference between the cost of the contract and its closing price on the day the contract is purchased. Post purchase, MTM margin covers the daily differences in closing prices .