Instead, it is mailed to the moderator. If the moderator thinks the article is suitable for the group, he puts it there for all to see. The moderator functions as a quality control unit. He makes sure only articles that belong in the group, get into the group. For an announcement group like c.o.l.a, this means that only announcements appear in the group.
Some announcements create a bit of discussion, and that discussion is redirected to another group.
Discussion, and stuff that is not related to Linux is out. Borderline cases include things like surveys and requests for testers. These are handled on a case by case basis. If you're unsure, ask the moderator. This was decided by a vote in 1995.