The category hierarchy tables have been replaced by m_hierarchy. This model defines named hierarchies of resources (pages).
If the categories are changed then the system needs to update the pivot_category_nr field of all resources. With the introduction of m_hierarchy this renumbering is much more efficient and will only affect a minimal number of resources.
The m_hierarchy module is also used for the content- and user group hierarchies, as used by the new mod_acl_user_groups module.