Users with the Admin role can always view all hierarchies and due to their super user permissions can view everything under those hierarchies.  All other users are assigned permissions to view a hierarchy via their role(s).

In the Role Service, when editing a role in the Role Manager, you can select which hierarchies are visible to that role.  A user will be able to see all hierarchies that are assigned to their role.

For more details on categories, roles, and permissions, refer to the “Authorization Management” section of the Niagara Station Security Guide