A margin call occurs when your available margin is less than 10% of used margin. When this happens, system will close your open trades one by one to bring available margin back above this 10% level, no matter their PnL. A margin call can significantly deplete your balance if the closed trades are big enough, which is why you should monitor your PnL and available margin closely.
System will notify you via email when your available margin hits 20% of used margin, and when the margin call actually occurs. However in a volatile market, warning might not arrive in time.
On the dashboard, there is something called Margin Coverage.
Margin Coverage here is the ratio between Equity and Used Margin. When this number reaches 110%, a margin call will occur. Equity being 110% of Used Margin is exactly the same as Available Margin being 10% of Used Margin.