Overview#Update Sequence Number (USN) is a 64-bit number in Microsoft Active Directory that increases as changes occur provided from Local counters on every Domain Controller.
At the start of each update transaction (originating or replicated) on a Domain Controller, the domain controller increments its current USN and associates this new value with the update request. This Update Sequence Number value is stored on an updated object in two ways for a replicated write and three ways for an originating write.
The update's Update Sequence Number value is stored with each attribute changed by the update as the local USN of that attribute (originating and replicated writes). You can use the Repadmin command-line tool to view the local USN. Type repadmin /showmeta <object_DN > at a command prompt and view the column labeled "Loc. USN" in the output.
The maximum local Update Sequence Number among all of an object's attributes is stored as the object's usnChanged attribute (originating and replicated writes).
For an originating write, the update's USN value is stored with each updated attribute as the originating USN of that attribute. Unlike the local USN and usnChanged , the originating USN travels with the attribute's value as it replicates.