In a clean Magento instance this layout does not exist. This is a feature that allows you to change the category layout as you want just by adding blocks to this layout handle. You can add something like this in one of your theme layout files.
<CATEGORY_6>
<reference name="content">
<block type="core/template" template="path/to/template.phtml" />
</reference>
</CATEGORY_6>
The layout above will add a block with the template path/to/template.phtml
on your category page (with ID 6).
If you want to take a look at what blocks are rendered in a page you can identify the default handle for that page like this. If the url to the page is module/controller/action/param1/value1/param2/value2
the layout handle for it would be <module_controller_action>
.
For categories there is an other layout that is loaded depending on how many child categories the main category has. If it has no children than this will also be loaded <catalog_category_layered_nochildren>
.
On other layout handle loaded depends on the is_anchor
attribute of the category.
This comes from $update->addHandle($category->getLayoutUpdateHandle());
And Mage_Catalog_Model_Category::getLayoutUpdateHandle()
looks like this:
public function getLayoutUpdateHandle()
{
$layout = 'catalog_category_';
if ($this->getIsAnchor()) {
$layout .= 'layered';
}
else {
$layout .= 'default';
}
return $layout;
}
If is_anchor
is Yes
then the layout handle added is catalog_category_layered
. Otherwise catalog_category_default
is loaded.
[EDIT]
Also, what you add to the category from the admin panel under Display settings
is added to the layout.