Cross Platform DLC

With the exception of 11.14 what you are proposing is most likely a violation of 11.1 or 11.2. Keep in mind, most people are either all iOS or all Android.

11.1 Apps that unlock or enable additional features or functionality with mechanisms other than the App Store will be rejected

11.14 Apps can read or play approved content (specifically magazines, newspapers, books, audio, music, video and cloud storage) that is subscribed to or purchased outside of the App, as long as there is no button or external link in the App to purchase the approved content. Apple will only receive a portion of revenues for content purchased inside the App

That would mean all the cross-platform collectible card games like Hearthstone, Cards and Castles and Solforge are in violation of the Apple terms. Clearly this is not the case or the apps would have been rejected.

I think 11.2 is banning the addition of links to other stores within the app, and doesn't necessarily ban unlocking content that had been purchased elsewhere.

11.2 Apps utilizing a system other than the In-App Purchase API (IAP) to purchase content, functionality, or services in an App will be rejected

I think you're right to mention 11.1 as potentially banning this practice. Does Apple consider, for example, the Battle.net account used for cross-platform deck ownership to be a violation of their terms? If not, why not? If it's against the rules why haven't these apps been banned? If it's NOT against the rules, what section of the "App Store Review Guidelines" allows it? Virtual cards are not among the "approved content" listed in 11.14. Hearthstone also allows you to purchase additional solo-play levels, both with in-game currency and with actual money. If I buy one of these levels with Amazon coins on my Android tablet, I can then play the level on my iPhone.

A couple of points: 1) they are 'guidelines' not terms and conditions. The terms and conditions are that Apple can accept or reject an app whether or not it satisfies the guidelines. 2) they are guidelines for rejection not acceptance. Don't look to them for 'a section that allows' something. 11.14 is a double negative (Not X will not result in not approval) which refutes the negative but does not assert a positive. 3) 11.2 remains ambiguous: "11.2 Apps utilizing a system other than the In-App Purchase API (IAP) to purchase content, functionality, or services in an App will be rejected" could be interpreted as 'apps using PayPal outside the app to purchase services et al delivered inside the app will be rejected' or it could be interpreted as 'apps using PayPal in the app to purchase services delivered inside or outside the app will be rejected.' I have no idea which is intended. 4) the multi platform issue creates a dilemma that involves a trust issue. If apple could trust a developer to fairly apportion the 30% margin then there is no problem. But what about the developer who is trying to divert purchases away from IAP to their own website. How does apple protect against that? Perhaps that's why they are guidelines that shift based on a perception of earned trust. All, IMHO.

Thank you. You give me hope that this will be possible, but without an authoritative answer from Apple I'm still left wondering if this is even worth pursuing. If the Ticket to Ride developers are to be believed, Hearthstone and all the other games that allow cross-platform DLC ownership of content not listed in 11.4 are in violation and in danger of being yanked any second.

I contacted Apple Developer support with my question. They told me to contact the App Review Team. The problem is, the link they gave me was to a page I can't access without having paid the $99.00 annual fee to be an Apple developer. This strikes me as a fairly basic question about apps that will support multiple platforms. apparently I can't even ASK Apple the question without paying first.

If I gave you hope that your approach would be acceptable then I apologize for your misreading.

You wrote:

"Users running the iOS version of my game will only be able to purchase maps through an Apple IAP."

Then you wrote:

"I would however like these users to optionally register for an account on a server I host and sync their owned maps across platforms. "

The first is not a violation of 11.1. It will work, hope is reasonable. The second is a violation of 11.1. It will most likely be rejected. Abandon hope. There is no need to ask App Review about this; 11.1 is clear, they will tell you 'no'. If you wish to inform Apple that there are Apps on the App Store that let you violate 11.1 you are free to use the Bug Reporter system to tell them. If you wish to take your chances that App Review will not realize that your system allows users to do this, go ahead.