@El-Gavy Good question, thanks for asking. The Cloudron backend is FOSS and usable completely standalone and you can use it without the fontend or the Cloudron App Store. It's actually very similar to other projects like dokku, flynn, deis, kubernetes, docker etc in that regard. These projects are also "backend" software and have no web interface to do stuff. To make the backend "useful" out of the box, our app packages are also open source.
The Cloudron frontend (web interface) makes use of the REST API and is part of our Open core style business model. The Cloudron App Store is a service for 1-click deployment and update of curated apps which is also part of our business model.
IOW, I like to think of Cloudron frontend as building on an opensource components akin to products like Docker Data Center and Tectonic (which are closed source).