This action allows you to define custom actions that you would like to perform on the selected file(s). In the field below you would enter your custom action - perhaps running Norton Anti Virus against the file(s) - by entering the command and the &f for the file name. So to run Norton AV - you would enter something like
NAV &f
This would then launch Norton Anti Virus with the correct fully qualified file name. The program will keep track of the last custom action entered and bring it up the next time you run this function.