The issue affecting app slicing has been fixed, and device-specific versions of apps will be delivered when downloaded by users running iOS 9.0.2 or later.App slicing is one facet of App Thinning, an iOS 9 feature designed to decrease the size of iOS apps to save valuable space on devices without a lot of storage space. With app slicing, users only download the app assets created for their particular device rather than a full set of assets for multiple devices. For example, on an iPhone 6s, a user will only need to download iPhone 6s specific files, ignoring both larger iPad artwork and lower quality resources for earlier iOS devices.
To take advantage of the space saving features in app slicing, all iOS 9 users should upgrade to iOS 9.0.2. App slicing is not available in earlier versions of iOS 9 or in iOS 8.