The tag can be used to retrieve the page with adw.navigation_view.NavigationView.findPage, as well as with adw.navigation_view.NavigationView.pushByTag, adw.navigation_view.NavigationView.popToTag or adw.navigation_view.NavigationView.replaceWithTags.
Tags must be unique within each adw.navigation_view.NavigationView.
The tag also must be set to use the navigation.push action.