Looking for explanatory for Precedence line

["Andrew Billinghurst" <billinghurst @ tpg . com . au>]

I have been asked some questions about the mailing list header line
 Precedence: ...
after having mail blocked by ISPs for use of the extension 
Precedence: junk and Precedence: bulk, both of which I understand are 
very appropriate for mailing lists.

Now I can find brief reference to their use in the new Sendmail book 
BUT I am unable to find any RFC for their use from first principles 
or other places where I can read about these header lines in depth.

Anyone got any pointers?

Thanks.  Regards, Andrew