Also known as symlinks, @links point to categories that could serve as subcategories of the current category. Symlinks are distinguished with the @ at the end of the category name, and there can be many @links in a single category, mixed in with the subcategories.
A new subcategory that might duplicate the entries of an already existing category should include instead an @link to that category, since having more than one category with the same listings means more work for editors and a less useful directory for users.
Example: Arts/Performing_Arts
Magicians@
This @link leads the user to Arts/Performing Arts/Magic/Magicians.
Thus, the Magicians@ category in the example is @linked to Magic/Magicians and Magic/Magicians is @linked from Arts/Performing_Arts.
Notice that some downstream data users do implement @links (and related category links), some don't, some do without the '@' symbol, thus displaying them as normal categories in the list.