The minimum size of the update sequence number (USN) journal to allocate on the volume, in megabytes. If no USN journal exists, ...

The minimum size of the update sequence number (USN) journal to allocate on the volume, in megabytes. If no USN journal exists, the DFS Replication service creates a new journal with the specified size; otherwise, the service increases the existing USN journal to  this minimum size. The DFS Replication service will not change the existing USN journal size if the existing size is already larger than the new minimum size.