Mastering Active Directory - Second Edition

The Update Sequence Number (USN)

The USN is a 64-bit number, which is allocated to the domain controller during the DCPromo process. When there is any object update, the USN allocated to the domain controller will be increased. As an example, let's assume that domain controller A had an initial USN value of 2,000 assigned to it. If we add 5 user objects, the new USN will be 2,005. This number can only increase; it cannot decrease. The USN is only valid for its own domain controller. It is not technically possible for two domain controllers in the site to have the same USN assigned.