you can use Objects themselves as your category folders (they don't have to be in-game rooms), in similiar usage to as 'storage' and~or 'data' Objects:
the in-game rooms that can be visited are the Objects: 'france', 'germany', and 'united_kingdom'
whereas the organizational 'folders' are the Objects: 'worlds', 'earth', and 'countries'.
<object name="worlds">
<object name="earth">
</object>
</object>
<object name="earth">
<object name="countries">
<object name="france">
</object>
<object name="germany">
</object>
<object name="united_kingdom">
</object>
</object>
</object>
and to add to the above, you can also do this too:
<object name="worlds">
<object name="earth">
</object>
<object name="mars">
</object>
</object>
<objects name="countries">
<object name="france">
<parent type="object">earth</parent>
</object>
<object name="martianopolis">
<parent type="object">mars</parent>
</object>
</object>