Page and Design Reference Providers
Available since version 1.6.0
Purpose
Help authors avoid stale content by prompting them to activate referenced pages and designs.

It is a known issue that custom reference providers, such as these, always show up under the “Unknown” category.
How to Use
Both components need to be enabled by creating sling:OsgiConfig nodes named com.adobe.acs.commons.wcm.impl.PagesReferenceProvider and com.adobe.acs.commons.wcm.impl.DesignReferenceProvider. There are not any configurable properties, so just an empty node will suffice.