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.
Read further here
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