Test data is copied to your live site when you publish

On the first publish, all records in Test data are copied to your live site. Later publishes add only new records, leaving existing live records untouched. This includes records created in the Test data tab or through the site; migration-created data is set up on live by the same migrations. For existing sites, copying begins when the sandbox picks up the new template, and records created before then aren’t copied. Owners of sites published before this update will see a one-time notice in the Publish dropdown, and the Data tab banner has also been updated.