When you create a work package, you group existing release under a new parent release. A work package can be useful, for example, when you have a release with 100 separate tasks and 90 of the tasks have been completed. You can group the remaining 10 tasks into a work package with a new parent release. The 10 tasks become tasks under the new release, and any release that are not tasks become children of the new release.
To create work packages, select the release, then use the Create Work Package action.