INTERNET-DRAFT                               Charles H. Lindsey
Usenet Format Working Group                  University of Manchester
                                             February 2000

6. Optional Headers

6.  Optional Headers
   The headers appearing in this section have established meanings and
   MUST be interpreted according to the definitions given here. None of
   them is required to appear in every article but some of them are
   required in certain types of article, such as followups. Any header
   defined in this (or any other) standard MUST NOT appear more than
   once in an article unless specifically stated otherwise.
   Experimental headers (4.2.2.1) and headers defined by cooperating
   subnets are exempt from this requirement.  See section 8 "Duties of
   Various Agents" for the full picture.