In tribe, admins can define if regular members can or cannot add topics to their contents. Also, admin can define if adding topic is mandatory or not.
Where you can define these options? Form Admin Panel> Content Type> turn on/off the content tagging settings from the right menu.
Admins and only the author of that post can edit or add topics assigned to that post.
Also, recently, Tribe added an option to the admin panel that allows admins to choose if the regular users in the community are able to create a new tag while posting a content or not.
Where to change this settings? Log in with your admin accounts> Admin Panel>Settings> Access> Who can create a topic> Save:
If you choose the option "regular users can create new topic", it means that when regular users are posting a content, if they want to add a topic to their content, they can search for the related topic. If the topic they are looking for does not already exists, they are allowed to add it as a new topic (Tag).
If you choose only admin or only admins and moderators can create new topics, it means that regular users should pick from the already existing topics when they want to add a topic to their contents.