A Mendix app model starts with a project. A project contains several top-level or project documents such as settings and security, and it contains the modules that make up a Mendix app.
Project-level documents describe project security, project settings, app navigation (for the different modes of navigation) and translations of the texts in your app.
A module consists of a domain model, model security settings and zero or more documents (e.g. microflows, pages).
Modules consist of documents, e.g. microflows, pages or published or consumed services. These documents can be organized with folders.
In this sense, Module and Folder behave in the exact same way, and thus inherit from FolderBase: they both contain documents and folders.