This new property is explained in the WWDC session "What's New in Core Location".
The default value is
NO if you link against iOS 9.
If your app uses location in the background (without showing the blue status bar) you have to set
YES in addition to setting the background mode capability in Info.plist. Otherwise location updates are only delivered in foreground. The advantage is that you can now have location managers with background location updates and other location managers with only foreground location updates in the same app. You can also reset the value to
NO to change the behavior.
The documentation is pretty clear about it:
By default, this is NO for applications linked against iOS 9.0 or later, regardless of minimum deployment target.
With UIBackgroundModes set to include "location" in Info.plist, you must also set this property to YES at runtime whenever calling -startUpdatingLocation with the intent to continue in the background.
Setting this property to YES when UIBackgroundModes does not include "location" is a fatal error.
Resetting this property to NO is equivalent to omitting "location" from the UIBackgroundModes value. Access to location is still permitted whenever the application is running (ie not suspended), and has sufficient authorization (ie it has WhenInUse authorization and is in use, or it has Always authorization). However, the app will still be subject to the usual task suspension rules.
See -requestWhenInUseAuthorization and -requestAlwaysAuthorization for more details on possible authorization values.