After cutover the new servers are used to send and receive mail and that allows you to start Differential data pull.
After initial data pull, during MX records propagation, with the MFR enabled, some new mail has been delivered to the old server. We will copy this data on this stage, Copy Remaining Exchange Mailbox Content:

The progress is displayed in the same way as it was for the first mailbox data import. You can Restart Data Transfer if there were connectivity issues during export and Details are also available for view.
When the differential data pull is finished, the Onboarding process comes to an end. You can mark the process finished and close the ticket by clicking Mark Onboarding as Completed on the next stage.