To become searchable, entries must be added to a search index. An NSUserActivity can be marked as either public or private. Public activities will be pushed up to Apple and facilitate app discovery. When the amount of public NSUserActivity records submitted from your app reaches a certain threshold, these search results will show up during search even if the user doesn’t have the app installed!
Probably due to deficiency on my part, but I’m having trouble imagining good examples of this and how it’ll work in practice.