Reference: FTP Server Administration options
The Data Port and Max Data Port fields define a range of ports for the server to listen on when performing passive mode data transfers. The server will select an unused port from this range and return it with the response to the PASV command. This allows the remote client to make an appropriate call. The server leaves the selection of ports up to the underlying network software if these fields remain 0.
A range is typically specified so that encrypted connections may traverse a firewall. In this case, you may also need to specify the ip address that is returned with the answer to PASV. This information is normally inspected and adjusted by firewalls and routers. This inspection is not possible when SSL connections are used.