If Cross Margin is selected, your account's entire available balance will be used as position margin. By default, all positions are initially set to “Cross Margin”. You can change it by adjusting the leverage with the slider located on the left-hand side of the trading page.
If you increase leverage, you reduce the amount of margin assigned to your position and that balance goes back to your “Available Balance”. Equally, if you decrease leverage, you increase the margin assigned to your position.
When using isolated margin, you can only lose the margin assigned to your position. Your available balance will not be used to add additional margin to your position in order to avoid liquidation. Isolated margin is useful when wanting to limit potential losses.
You also have the ability to add and remove margin to/from your position. First, click the +B icon next to the margin line-item on an isolated position in the Positions List:
Then you can set the amount of margin to add or remove: