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In the DMOZ categories, sometimes, some categories has an "@" in front of them.
I notice this categories are somehow listed under more than one place. Is this what the "@" stands for?
What is the proper name to the categories that uses the "@"?
In effect all it really means is that the editor of the page you are looking at cannot edit the @ pages (but can only edit pages below the page you are looking at, that are directly "down" from that page.
In other words don't worry about it ;)
Yes, @linked categories always appear in at least two places in the ODP category hierarchy. All but one of those places contain the "@" character in front of the name.
You'll see similar concepts in other hierarchical taxonomies like the librarians' Dewey Decimal System and Library of Congress system.