The "app slicing" or "app thinning" feature Apple promised to launch with iOS 9 can help 16GB and 32GB iPhone owners save some precious storage space. Unfortunately, Cupertino can't release it to the general public yet (even though its beta testing app has been supporting the feature for quite some time), "due to an issue affecting iCloud backup." See, Apple gave developers the power to mark parts of their applications meant for specific devices. That means folks with an iPhone 6s will only end up downloading app components meant for the 6s and not the ones meant for iPads or other iPhone variants -- hence, the term app slicing/thinning.Unfortunately, due to the iCloud bug, iOS 9 users are still getting universal versions of apps instead of optimized ones. The note posted on Apple's developer website says the company will roll out app slicing with a future software update, though -- hopefully sometime soon, for the sake of everyone quickly running out of storage.
App slicing is currently unavailable for iOS 9 apps due to an issue affecting iCloud backups created from iOS 9 where some apps from the App Store would only restore to the same model of iOS device.
When a customer downloads your iOS 9 app, they will get the Universal version of your app, rather than the variant specific for their device type. TestFlight will continue to deliver variants for your internal testers. App slicing will be reenabled with a future software update. No action is needed by you at this time.