I expect the trusted authorities would be selected by the server where the user account resides. I.e. if a server’s admin does not recognize a certain authority, it would not show their verifications to users logged in to their server.

It’s possible that it could extend to user selections of trusted verifiers as well, but I think implementing that level of granularity would be more of pain than it’s worth to Bluesky. Still, I could be surprised.