Would I be correct in assuming that a user of my iOS app that uses CloudKit would not need to pay a monthly fee to Apple to use my CloudKit app? Apple just changed from annual to monthly for anything above 5GB for iCloud... or is it unrelated? I've been hunting around Apples development site and I cannot seem to find a reference for who gets charged a fee for use of CloudKit. WWDC made brief reference that there is a limit that can be reached and it seemed like they were indicating that the developer would have to pay Apple if their user base exceeded a certain point. If that's the case, a developer would have to be very careful to make sure they charge enough for the app. It could get expensive if there is a sudden increase in app use by a smaller number of users.
When you store data in the public database, then it counts as data for your app. If you store data in the private database, then it's counted at the user's account (now max 5GB for free). If you want to know how much data is available for free for your app, then have a look here: https://developer.apple.com/icloud/documentation/cloudkit-storage/
来源：https://stackoverflow.com/questions/25940738/icloud-versus-icloud-drive-versus-cloudkit