DisconnectNotifyPeer is an overridden method that handles closing the FTP session for the client. 

When the IOHandler is still connected to the remote FTP server, DisconnectNotifyPeer sends the FTP QUIT command to indicate that the client will be closing the control channel for the FTP session. DisconnectNotifyPeer then waits for a brief period (10 milliseconds) for the IOHandler to become readable and to handle the response for the FTP QUIT command. 

Please note that the Quit method has been deprecated in Indy 10. 

Use Disconnect to simply close the control channel for the FTP session.