At the time of this writing, Privacy and, therefore, private data, are applicable only to Virtual Appliances.
Once a Virtual Appliance has been marked as Private, the following metadata become available only to the people that have the access private data permission set.
List of Virtual Appliance Private metadata:
If a Virtual Appliance is marked as Private, its image list is also handled as private. In case one would like to subscribe to a private image list, then he/she should follow this guide.