In order to successfully send a derivatives order you must have sufficient Margin Assets Value to cover the required Initial Margin. Therefore, the Initial Margin requirement of your new order cannot be greater than your current Available Margin. The amount of excess Margin Assets Value you have to be able to send a new order is shown by your Available Margin. 

If your current GUSD balance is 1,000 and your Initial Margin requirement due to open positions is 600 GUSD, then your Available Margin is 400 GUSD. If you try to send an order with an Initial Margin requirement of greater than 400 GUSD, it will be rejected. The maximum notional order sizes you can buy or sell are shown by the Buying Power and Selling Power values on the order entry form.