This option is designed for risk management.
If you set some value here then the lot size of each copied order will be adjusted and this order will risk no more than selected % of your account in case the Stop Loss is hit.
Note: the original order should have the Stop Loss level. 
To activate this setting please open Forex Copier 2 program, click “Settings” on Receiver account, go to “Lot/risk management” tab, activate “Use custom lot size” option,
then select “Use risk management based on Stop Loss” and enter necessary value there.
After this, click “Save and Finish” and then click “Apply Scheme” button to apply these new settings.