The drawdown limit is intended to be a limit on the total amount of your unrealised losses so if you take all of the open positions for this expert and you add all of their profits and all of the losses then that would be the drawdown that you have for this expert.

When the calculation of the total drawdown is made remember that this only applies to the open positions for this expert. the expert will use a combination of the current symbol and the magic number to determine which trades belong to this expert. see the notes on magic numbers for more information about this.

The drawdown limit is similar to a stop loss but if the expert is able to open more than one position of a time a stop loss would apply to each individual position separately where the drawdown limit applies to the total unrealised profit or loss for this expert.

 when the draw down limit is reached for an expert the expert will first close all open positions and orders for these expert. Then if you have selected the option to terminate the expert it will remove itself from the chart. If you have not selected the option to terminate the expert then trading will continue as normal.

Due to slippage and timing it is possible that you will lose more or less than the actual drawdown limit on the positions for this expert. Also, if the drawdown limit is triggered close to a time where the market will shut down it is possible that the expert will be unable to close those positions if the market closes before the activity to actually close the positions happens.

In most cases where an expert implements the drawdown limit there will be the following inputs

Use Drawdown Limit	Set this to true to use the drawdown limit feature. See here for more information on drawdown limits.This feature can be used even if a stop loss is applied because the stop loss applies to each trade individually where drawdown limit applies to the total unrealised loss of all trades opened by the expert.
Maximum Drawdown	The maximum drawdown limit in your deposit currency.
Shut down expert on drawdown limit	Set to true if you want to close this expert when the drawdown limit is reached. Shutdown will happen after orders and positions are closed, the expert will be removed from the chart.