Project extensions have very similar access control features to projects. However, in an extension, users with view only access can still contribute to tags and fields in the extension. This allows you to open up contribution, without giving everyone the ability to modify the tags or fields themselves (e.g. change the name or create new ones).
To control access to an individual extension:
Open the extension.
Lock your taxonomy
A common use case for extensions is to create a global taxonomy of tags and fields. You can allow people to contribute to your taxonomy, while disallowing the ability for them to modify the tags or fields themselves. In other words, you can ‘lock down’ management of the taxonomy while still allowing people to contribute to it.
To do this, set the workspace access in the extension to View only. This will allow people to extend their project with the extension (and thus contribute to the tags and fields in the extension), without actually being able to modify the tags and fields.
Must be available to all
Extensions must be viewable by everyone in the workspace, and cannot be hidden from the workspace by setting workspace or user access to No access.