In the derivatives space, margin refers to the amount needed to enter into a leveraged position.
Initial and Maintenance Margin refer to the minimum initial amount needed to enter a position and the minimum amount needed to keep that position from getting liquidated.
Cross Margin: Margin is shared between open positions. When needed, a position will draw more margin from the total account balance to avoid liquidation.
Cross Margin is a margin method that utilities the full amount of funds in the Available Balance to avoid liquidations. Any Realized PNL from other positions can aid in adding margin on a losing position.
This margin method is useful for users who are hedging existing positions and also for arbitragers that do not wish to be exposed on one side of the trade in the event of a liquidation.