As detailed in About the replication protocol, the Directory Server assigns a unique change number to each update operation, specifically ADD, DELETE, MODIFY, or MODIFY DN operations, to track each request received from a client. The change number is attached to each update propagated through replication and allows each Directory Server to order updates exactly the same way.

The change number is composed of the following fields:

Identifies when the update was made. The timestamp is time-zone-independent and recorded with millisecond resolution.
Server ID
Uniquely identifies the Directory Server where the update was made.
Sequence number
Defines the order of the updates received from external clients at a particular directory server.

The replication protocol sets a virtual clock that eliminates the need for synchronized time on servers. For troubleshooting purposes, you should keep the system clocks synchronized.