I use package name as the authority http://app-uri.sysapps.org/#dfn-authority
On Android app://<relative_path> is enough to meet the requirement from implementation side, as Android app is isolated with each other. But on other platforms each app should have an unique identifier, like the app ID of chrome app, in spec it defines it as the host name of URI. Also it needs support such case
console.log(loc.host === "com.foo.bar"); //true
So we need keep aligned with the spec and use the authority segment. But to simplify this I use the package name as it's unique on Android. On some other platforms, it uses the hash value of the directory.
The spec does clarify whether the resource can be accessed crossing the different regions. Maybe it will if there is some requirement of cross-region access.