Note 
Select Default Settings to let the operating system choose an available port. Content Gateway sends 0 and retrieves the new port number if the listen succeeds.
Select Specify Range if you want the listening port to be determined by the range of ports specified in the Listening Port (Max) and Listening Port (Min) fields.
Select Proxy Sends PASV to send a PASV to the FTP server and let the FTP server open a listening port.
Select Proxy Sends PORT to set up a listening port on the Content Gateway side of the connection first.