This feature is available in Enterprise and Ultimate editions.
Backlog Groups can be closed when all stories/backlog items within the Backlog Group have been completed and no more will be created. Used in this way, Backlog Groups can represent an evolving set of functionality within the system, each piece with its own lifespan of development. Closed Backlog Groups can always be reopened if needed.
Some organizations may never close Backlog Groups. A Backlog Group such as 'Reporting' may always be available for further development. The decision to close Backlog Groups primarily depends upon the structure and use of Backlog Groups within the organization: whether they represent ongoing areas or specific, time-boxed functions.