What does startaccessingsecurityscopedresource() mean for different OS?

So, I'm looking into startaccessingsecurityscopedresource() function and from my current understanding this is to get temporary access to files/folders you don't implicitly have access to i.e., that don't belong to your sandbox. I can understand what it means wrt macOS, iOS, iPadOS, but what does it mean in watchOS and tvOS where there isn't any file sharing between different apps? And what is it's relevance wrt using iCloud (if there is any?)