Most of what follows I learned recently by experimentation and by studying
the LSTSRV-L archives from when the stuff was announced.
 
You can have in the list header Topics= and Default-Topics= keywords.  Up
to twelve topics can be defined; there are also two reserved "topics".
 
A sender indicates one or more topics by putting them at the start of the
subject followed by a colon.  For example, in the following subject line,
the topics are "young" and "montana"
 
Subject: young, montana: That was then, this is now
 
Notes that don't indicate a topic will be treated as if their topic is
"Other" (this is one of the keywords).  The other keyword is "All." When
used as a note's topic, it means, "Send this to everyone who's signed up
for one or more topics."
 
A list owner can assign through Default-Topics a set of topics people will
get by default; users can add or drop topics through
 
set listname topics= +topic1 -topic2 topic3
 
where "topic1" would be added to the topic list and "topic2" and "topic3"
would be dropped for that user.  Thus, if a user has asked only for topics
"deberg" and "brodie" they won't get the above note.  A user can add or
drop "other" just like any normal topic.  "All" on the other hand means
each topic on the list; +all means a user gets all mail, -all means they
get no mail until they add one or more topics to the list.
 
set listname topics= brodie montana
 
would be equivilent to: topics = -all +brodie montana
 
List owners can change the topics, but because topics are stored by
relative values, not the real value of the word, on a user-by-user basis,
this can have weird side effects if you, for instance, change "brodie" to
"walsh"; suddenly the Brodie fans find their mail treated as "Other" and
get mail meant for "Walsh" fans.
 
Hope this helps,
Nick
 
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