Each forum has a specific use.
[url=http://www.bungie.net/Forums/topics.aspx?forumID=6]New Mombasa[/url] is for everything Halo 2 related.
[url=http://www.bungie.net/Forums/topics.aspx?forumID=7]The Library[/url] Is for all Halo X-Box topics.
[url=http://www.bungie.net/Forums/topics.aspx?forumID=2]The Maw[/url] is the official Halo PC forum.
[url=http://www.bungie.net/Forums/topics.aspx?forumID=3]The Septagon[/url] is the place for discussions about the Bungie community and The Seventh Column.
[url=http://www.bungie.net/Forums/topics.aspx?forumID=1]The Underground[/url] is where you should talk about Bungie itself or any old school Bungie games.
[url=http://www.bungie.net/Forums/topics.aspx?forumID=10]And The Flood[/url] covers anything that doesn't fit in any of the above categories.
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I don't know on this one. I always thought that it was best to answer the question asked by the thread-starter first, so he doesn't post the question in a wrong forum again. After you have done that, just notify a mod of the thread and have it removed.