The connection-mode (passive or active) of data-channels.
When the connection-mode is active, the server will initiate connections to the FTP client, meaning that the FTP client must open a socket and wait for the server to connect to it. This often causes problems if the FTP client is behind a firewall.
When the connection-mode is passive, the FTP client will initiates connections to the server, meaning that the FTP client will connect to a particular socket on the server. This is generally used if the FTP client is behind a firewall.