Contract the sub-collections menu.

Select all.

Add selected documents to current sub-collection. The default sub-collection
is the one that was last accessed or created.
To change the current subcollection or create a new one click here.

To open a subcategory, place your mouse over the respective category and click it once.
If you tick the checkbox in front of a category, all the documents under the respective
category will be selected for addition to the sub-collection. If you tick the checkbox in
front of a main category to add it to the sub-collection, all the corresponding subcategories
will be selected automatically as well.