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
In order to run the Initial synchronization go to profile view, click the cog icon and select "Start Initial Sync":
A dialogue window will appear and you'll be able to select:
Start date (issues created starting from this date will be synced)
Sync direction - indicating whether the issues will be migrated from Jira to Azure or from Azure to Jira
Once the choice is made, you can proceed with the initial Sync.
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.