rfc1036 December 1987

2.2.5.  References

    This field lists the Message-ID's of any messages prompting the
    submission of this message.  It is required for all follow-up
    messages, and forbidden when a new subject is raised.
    Implementations should provide a follow-up command, which allows a
    user to post a follow-up message.  This command should generate a
    "Subject" line which is the same as the original message, except
    that if the original subject does not begin with "Re:" or "re:", the
    four characters "Re:" are inserted before the subject.  If there is
    no "References" line on the original header, the "References" line
    should contain the Message-ID of the original message (including the
    angle brackets).  If the original message does have a "References"
    line, the follow-up message should have a "References" line
    containing the text of the original "References" line, a blank, and
    the Message-ID of the original message.

    The purpose of the "References" header is to allow messages to be
    grouped into conversations by the user interface program.  This
    allows conversations within a newsgroup to be kept together, and
    potentially users might shut off entire conversations without
    unsubscribing to a newsgroup.  User interfaces need not make use of
    this header, but all automatically generated follow-ups should
    generate the "References" line for the benefit of systems that do
    use it, and manually generated follow-ups (e.g., typed in well after
    the original message has been printed by the machine) should be
    encouraged to include them as well.

    It is permissible to not include the entire previous "References"
    line if it is too long.  An attempt should be made to include a
    reasonable number of backwards references.