Thanks Robert and Simon for the replies. For the permission approach, it's not quite applicable to me, because my sites can be seen by general public. All I want to do is when someone reading department A site , then the forum messages for department B should not appeared. So it's more like classification into departments. (Sorry for I didn't make it clear enough)
And yes, it is like what dotLRN does I guess. There is no reason why I can't use it, just that I didn't know how to do it :) Anyway, maybe I should download dotLRN later and see what they do with it.
Paul
