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Whenever an object is changed, its USN is incremented. When replication occurs, only the version of the object with the greatest USN is retained.
Local counters for USNs are considered reliable because they never decrease or "run backward." USNs are also always unique, making it easier for domain controllers to never use the same USNS at the same time.
One reason why experts prefer USNs over time stamps is because clocks are difficult to keep synchronized and because clocks can't account for any latency between network segments.
USNs can also be used in domain controllers to ease recovery if a failure occurs. When domain controllers are restored, they query replication partners to look for changes to the USNs that are higher than the USN of the last change each partner received.
One drawback of using USNs is that administrators can't compare USNs on one domain controller to USNs on other domain controllers. According to Microsoft, the system for replication was designed with this limit in mind.