The identifier for the shared container into which files in background URL sessions should be downloaded.
- iOS 8.0+
- macOS 10.10+
- tvOS 9.0+
- watchOS 2.0+
To create a URL session for use by an app extension, you must set this property to a valid identifier for a container shared between the app extension and its containing app.
For information about app extensions, see App Extension Programming Guide.