2.2.13.  Xref

    This line contains the name of the host (with domains omitted) and a
    white space separated list of colon-separated pairs of newsgroup
    names and message numbers.  These are the newsgroups listed in the
    "Newsgroups" line and the corresponding message numbers from the
    spool directory.

    This is only of value to the local system, so it should not be
    transmitted.  For example, in:

               Path: seismo!lll-crg!lll-lcc!pyramid!decwrl!reid
               From: reid@decwrl.DEC.COM (Brian Reid)
               Newsgroups: news.lists,news.groups
               Subject: USENET READERSHIP SUMMARY REPORT FOR SEP 86
               Message-ID: <5658@decwrl.DEC.COM>
               Date: 1 Oct 86 11:26:15 GMT
               Organization: DEC Western Research Laboratory
               Lines: 441
               Approved: reid@decwrl.UUCP
               Xref: seismo news.lists:461 news.groups:6378

    the "Xref" line shows that the message is message number 461 in the
    newsgroup news.lists, and message number 6378 in the newsgroup
    news.groups, on host seismo.  This information may be used by
    certain user interfaces.