Initial synchronization gives you possibility to synchronize (migrate) Jira Issues and TFS / Azure DevOps (formerly VSTS) Work Items that were created before TFS4JIRA Synchronizer has been installed.
Starting the Initial Synchronization
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At this point, the initial sync will begin in 15 seconds and the issues starting from selected date will be moved to the corresponding project.
Once the Initial Sync starts you will see the synchronization status and ultimately - an Initial Sync summary, showing how many items were synced succesfully and if there were any errors:
You can view the details of initial sync clicking "See Details" button in Profile Events