The below is from the user guide:
"If the domain controller being recovered is the only functioning domain controller in the domain, a primary restore of the SYSVOL should be done. A primary restore builds a new FRS (File Replication Service) database by loading the data present under the SYSVOL onto the local domain controller. This method is the same as non-authoritative except that the restored data is marked as the primary data.
Perform a primary restore only when all domain controllers in the domain are lost and you want to rebuild the domain from backup. Do not perform a primary restore if any other working domain controller in this domain is available. Use primary restore for the first domain controller, and then, later, use non-authoritative restore for all other domain controllers. The restore operation is not completed until the target domain controller is restarted in normal operational mode."