What is a "Hidden" project?
From Planfix
You can regulate which project names are visible in project lists for employees and external contacts with Planfix access:
Keep in mind that even if a project doesn't have the "Hidden" status, project tasks are only visible to people with access. Other users can see the name of an open project in the project list, but they can't see the project's tasks.
Hidden from employees
By default, all projects are visible in the project list to any account employee. By setting a project to "Hidden," we allow only the employees who are participating in it to see it in the list (employees who have access to at least one project task). This can be useful when employees not involved in a project don't need to know that it exists.