Margin Call is a notification which lets you know that you need to deposit more money in your trading account in order to sustain your open trades, or close losing positions, so you free up more margin. It's widely known as a fixed percentage.
For Naga the first warning email comes when your margin drops below 100%. Your trades will start closing when your margin has reached the 50%, starting with the most unprofitable one.
Please check our educational video here, as an example is given along with the explanation.