Sent by a client that wishes to disconnect from a host, before actually going ahead and severing the connection. This gives the host a chance to gracefully free any resources dedicated to this client, rather than having to find out after the fact that it has disconnected.
This command needs to be sent even when there is no established connection, so it also requires the address of the machine to send it to. This information is implementation/API-level though, it does not appear in the command's network representation.