D2 and D4 are used to restore a SYSVOL Replica Set in Active Directory domain. The D2 is generally called Non-Authoritative and D4 is called Authoritative. These two terms are used by the File Replicatoin Service and set in registry keys of the domain controllers. When these two values are used in an Active Directory domain, the DC on which you have set the D4 Flag is called the Authoritative Server for the SYSVOL Replica Set and acts the Primary Member for D2 DCs. The D2 DCs will then sync the SYSVOL Replica Set contents when you restart the FRS service on these computers. They will contact the D4 DC to compare and sync the changed files. This process is called D4-D2 restore.
When should you use them?
Don't just go ahead with D4/D2. D4/D2 shall be the last resort except the followings:
- FRS computer is throwing JOURNAL WRAP Errors in the FRS Event Log
- Junction Points have been changed or deleted
- File/folders are morphed
For troubleshooting FRS and when you to use D4/D2, please check the following Knowledge Base at Microsoft site: