rfc1036 December 1987

2.2.6.  Control

    If a message contains a "Control" line, the message 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.
    Also for upward compatibility, if the first 4 characters of the
    "Subject:" line are "cmsg", the rest of the "Subject:" line should
    be interpreted as a control message.