usefor-article-03 February 2000

4.2.2.3.  Local Headers

   Headers with the local property are significant only to a particular
   serving agent (or perhaps a cooperating group of such agents). They
   MAY be removed by relaying agents before propagation, and MUST be
   removed (and replaced as necessary) by serving agents when received.
   The replaced header MAY be placed anywhere within the headers (though
   placing it first is recommended). The principle example is:
     o Xref (6.14) - used to keep track of the article locators of
       crossposted articles so that newsreaders can mark such articles
       as read.