What should be the expected user experience from an app that uses CloudKit/Sync

The following is an opinion. Informed, but still an opinion.

1a. If your app's core functionality requires iCloud and the user is not signed into iCloud, display a clear message telling the user that they'll need to sign in to iCloud to use your app and include a button to take them to the system page for signing in. Do not simply display an alert, but rather show it in an attractive, polite, and modeless/full-window presentation.

1b. If your app includes a non-essential feature that requires iCloud and the user is not signed into iCloud, display a modeless notification/affordance to indicate that the feature is inactive and will remain so until they sign into iCloud. The notification might take the form of a special icon positioned where that feature is normally accessed. Tapping that icon (or hovering, on a Mac) might pop up a brief explainer. If the feature's importance warrants it, a more prominent notification might be used. Modeless is better than modal.

1. Sync should be automatic. Users will expect this if they use your app across multiple devices. Making it automatic may also clarify what you choose to sync. User data should certainly sync, but user settings may not. If I set my background color to blue to match my blue iPhone, I might not appreciate that my silver iPad's background color is now blue as well. Resist the temptation to ask the user whether they'd like to sync this or that. Rather, make that determination yourself after carefully considering the value to the user.

Also, if possible give the user a way to initiate a manual sync—it's reassuring—and display an indicator like a green checkmark to indicate that the sync completed successfully or failed. One common way to provide a manual sync is by including a pull-to-refresh on the scrollview containing the user's content.