This is an optional property. An app that is also a client of iCloud Drive can set this property to allow Spotlight to deduplicate its searchable items against the iCloud Drive items. In this scenario, Spotlight does not display searchable items from iCloud Drive that have the same contentURL property.
For activities, the unique identifier for the item to which the activity is related, but not linked.
This documentation contains preliminary information about an API or technology in development. This information is subject to change, and software implemented according to this documentation should be tested with final operating system software.