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.
The Control header content marks the article as a control
message, and specifies the desired actions (other than the
usual ones of filing and passing on the article): Control-content = verb *( space argument )
verb = 1*( letter / digit )
argument = 1*<ASCII printable character> The verb indicates what action should be taken, and the
argument(s) (if any) supply details. In some cases, the
body of the article may also contain details. Section 7
describes the standard verbs. See also the Also-Control
header (section 6.15). NOTE: Control messages are often processed and
filed rather differently than normal articles. NOTE: The restriction of verbs to letters and dig-
its is new, but is consistent with existing prac-
tice and potentially simplifies implementation by
avoiding characters significant to command inter-
preters. Beware that the arguments are under no
such restriction in general. NOTE: Two other conventions for distinguishing
control messages from normal articles were for-
merly in use: a three-component newsgroup name
ending in ".ctl" or a subject beginning with
"cmsg " was considered to imply that the article
was a control message. These conventions are
obsolete. Do not use them. An article with a Control header MUST not have an Also-
Control or Supersedes header.