The opposite of on-topic is off-topic. A contribution may be off-topic with regard to a thread/page/etc., or with regard to the whole site/newsgroup/etc., which is often established for dealing with specific topics.
Being on-topic is not generally required to be indicated, but in some groups on-topic discussion is so rare that the header is marked accordingly.
_______________________________________________ Grupo de Usuários do PostgreSQL no Brasil http://www.postgresql.org.br
