If an article contains a Control line, the article is a control message. Control messages are used for communication among USENET host machines, not to be read by users. Control messages are distributed by the same newsgroup mechanism as ordinary messages. The body of the Control header line is the message to the host.
For upward compatibility, messages that match the newsgroup pattern "all.all.ctl" should also be interpreted as control messages. If no Control: header is present on such messages, the subject is used as the control message. However, messages on newsgroups matching this pattern do not conform to this standard.