Persistence of photo access permission ?

Can you clarify if this question is about the Limite Photos Library or PHPicker?

When a user extends the Limited Photos Library, the access is persisted until they change the selection.
When your app has the authorization status PHAuthorizationStatusLimited the assets you can access will stay the same until the user changes it.
Generally it’s not recommended to persist the local identifiers, since they may change under certain circumstances.

What do you mean with "it’s not recommended to persist the local identifiers, since they may change under certain circumstances"? Can you go a little deeper on "certain circumstances"?

Apologies on the typo, I was asking about PHPicker. I recall something about a cloud identifier.. should that be used for persistent storage in the app context in place of local identifier?
My app basically stores metadata of the ciFilter set and the image local identifier but does not save a new image.
This is because localIdentifier is documented as “a unique string that persistently identifies the object” (class PHObject). Is this changing too?

Sorry for jump into this thread started  and keep hammering on the local identifier. As mentioned by Will Loew-Blosser, local identifier was reported as being unique. Now let's taking iMovie as an example. This app, as many other video apps, do have the concept of project and a project is a configuration that can reference assets that were unique.
The only thing that you had to handle was the possibility for an asset to be deleted when the project was used between sessions.
If the asset does not have a unique identifier anymore, how to handle a scenario in an app that uses the concept of projects referencing assets?