On the 'Assets' tab, the 'Used Margin' is the current total isolated margin used for:
1. All open positions including a fee to close positions
2. All active orders including 2-way taker fee.
Used margin = Positions margin + Order Margin
Positions Margin= Used margin of positions + fee to close
Order Margin = Used margin of open order + 2-way taker fee (fee to open + fee to close)