Right, staff can move tickets to hidden departments but they can not manage tickets from those departments. It's a good option to assign ticket to a private group.
Thanks and sorry for the slow reply! I'll explain my use case a bit.
I have various different organisations working with me via a ticket system. Each organisation is separate, and only the admin (me) should be able to move tickets between organisations.
Each organisation has its own group of ticket "departments" that their tickets can move between (it's a custom workflow, in effect - workaround approach to custom fields).
I'd like to restrict each organisation to seeing only its own departments in the selector.
This is both to avoid confusion / overly long select boxes, and so that tickets can't accidentally be moved to a different organisation.
(I have to be careful about GDPR, personal info etc.)
I realise this may be an obscure use case!
I'm a PHP dev but not familiar with the MVC framework you're using - happy to customise my own version given a steer towards the core files to modify.