An update sequence number (USN) is a 64-bit number in Active Directory that increases as changes occur. Local counters on every domain controller assign USNs.

An update sequence number (USN) is a 64-bit number in Active Directory that increases as changes occur. Local counters on every domain controller assign USNs. 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.