iCloud allows you to back up and synchronize files on all connected user devices. With Realm, you could theoretically agree to this if only one client accesses the data at any given time, which will be difficult to guarantee. In addition, there are file size restrictions that make this solution vulnerable.
CloudKit is a backend service solution that allows you to customize the circuit and request data from you. It does not bring a built-in save layer. You will need to provide this logic yourself, for integration with CoreData as for Realm.
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