Binary Rejected – in essence because the IAP must be available on all devices.
What this means, essentially, is that you need to register the user and then use server authentication (i.e. record an IAP) to determine whether the user can access that content on another iOS device. Note that this only applies to subscriptions – not consumable products.
So, a subscription purchase should be available to a user on any device they have. I.e. you don’t get to charge a user twice if they have two devices!
To be absolutely clear – there is no such requirement for consumables. Consumables can be for just the device the user bought them on. To make consumables be cross-device, you’d need to implement it yourself with iCloud or other technology.
The full text of 11.6.
11. Purchasing and currencies
11.6: Content subscriptions using IAP must last a minimum of 7 days and be available to the user from all of their iOS devices
We found that while your app offers a content subscription, there is no mechanism in place to support the App Store Review Guidelines requirement that the subscription content be available to the user on all of their iOS devices.
The detailed requirements for the Subscription Purchasability type are available in the Entering In App Purchase Information section of the iTunes Connect Developer Guide:
“…subscriptions must be provided on all devices associated with a user. In App Purchase expects subscriptions to be delivered through an external server that you will provide. You must provide infrastructure to deliver subscriptions to multiple devices.”
It would be appropriate to modify your app to include an optional user registration feature, to deliver subscription content to all of a user’s iOS devices. Such user registration must be made optional, not required. We also recommend indicating that registering is required to access the subscription content from their other iOS devices – and providing a way to register later, if users wish to have access to this content at a future time.
It would also be appropriate to advise the user that the Premium purchase is a recurring subscription, and not a one-time payment.