The backup service covers all family trees on the specific family site for which the service has been acquired.
The service does not include content in the family site such as comments by other users, Inbox messages, Smart Matches, Charts and Reports or any content other than family tree data, photos, videos and documents that were tagged to the family tree.
The Backup service provided for Family Tree Builder trees excludes any photos not associated and synchronized in the family tree. Photos uploaded directly to a family site, but not tagged to the synced tree, will not be covered by the backup.
To subscribe to the backup service for your family trees, please read the following FAQ article: How can I purchase the backup service?