The Estimated Liquidation Price is shown in the Order Entry panel on ActiveTrader. This is the price at which it is estimated the Margin Assets Value of the account will fall below the Margin Maintenance Limit and result in the liquidation process being run. 

If the account has more than one perpetual position then it is assumed that there is 100% correlation between assets in order to calculate this price. The price is calculated by looking at the loss that can be sustained in the account and what price movement for the assets would trigger such a loss.