Here is a link
https://developer.apple.com/library/ios/documentation/DataManagement/Conceptual/DocumentBasedAppPGiOS/ManageDocumentLifeCycle/ManageDocumentLifeCycle.html#//apple_ref/doc/uid/TP40011149-CH4-SW1
Setting the Preferred Storage Location for Document Files
All documents of an application are stored either in the local sandbox or in an iCloud container directory. A user should not be able to select individual documents for storage in iCloud.
When an application launches for the first time on a device, it should do the following:
If iCloud is not configured, ask users if they want to configure it (and, preferably, transfer them to Launch Settings if they want to configure iCloud).
If iCloud is configured but not enabled for the application, ask users if they want to enable iCloud—in other words, ask if they want all of their documents saved to iCloud. Store the response as a user preference.
Based on this preference, an application writes document files either to the local application sandbox or the iCloud container directory. (For details, see “Moving Documents to and from iCloud Storage.”) An application should expose a switch in the Settings application that enables users to move documents between local storage and iCloud storage.