Ok, that makes sense. Thanks fellas (and/or ladies) !
When in doubt “Google”? I copy this from the Doom9 Forum, sounds valid.
The CERTIFICATE folder should always exist on a bluray whether empty or not
If the disc contains BD-J (JAR files) and they are signed, then there needs to be a certificate
in that folder that authenticates the signature of the JAR files
If the JAR files are not signed, there is no need for a certificate, but BD-J then cannot access
some privileged functions, among them access to persistent storage or access to the internet
If the certificates are removed and a signed JAR file wants to access privileged code, the disc
simply will not play.
If BD-Live content is downloaded from the internet, it may (usually is) signed too, requiring a certificate in that very folder to ensure, that the downloaded content can’t be tampered with.