The update sequence number (USN) is a 64-bit number, which is allocated to the domain controller during the Dcpomo process. When there is any object update, the USN allocated to the domain controller will be increased. For example, let's assume the domain controller A had initial USN 2000 assigned to it. If we add five user objects, the new USN will be 2005. This number is only increases. It cannot be decreased. The USN is only valid for the its own domain controller. There is no technical possibility for two domain controllers in the site to have the same USN assigned.