First we will see what is USN? Microsoft says..

Every domain controller maintains a USN that is specific to that domain controller. Whenever a change is made to Active Directory from that DC, the USN is incremented by 1. So if a DC has a USN of 1000 at 11:00 a.m., and 1005 at 11:30 a.m., you know that 5 changes have been made to the Active Directory database on that DC. Exactly what these changes were is unimportant as far as the USN is concerned—you could have modified 5 different objects, created 5 objects, deleted 5 objects or any combination thereof, the DC’s USN will still increase by 5. Moreover, USNs are internal to a specific domain controller only, and don’t have any relevance when compared with other DCs. One DC in a domain might make a change at 11:30 a.m. that it assigns a USN of 1051, and a second DC in the same domain might make a change at precisely the same moment that it assigns a USN of 5084. While these two DCs clearly have radically different USNs for a change made at roughly the same time, this fact is irrelevant to how these changes are replicated; the Update Sequence Number of one DC has no meaning to any other DCs in terms of comparing one change to another.

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Verifying USN number are always helpful and give some clue when you are troubleshooting a replication issues.

Let’s see how to check the current USN on a domain controller.

Run the below command on the Powershell

repadmin /showutdvec PDC01 dc=renjithmenon,dc=com