Is CPS really open source? Aren there any proprietary extensions that are sold on top of the platform?
CPS is 100% open source / free software. You can use it, modify it and redistribute it, under the terms of the GPL license. We (Nuxeo) don’t sell licenses, only support and develop customers projects on top of CPS Platform. And if you are a free software developer, you are very welcome to join the project as a tester, bugfixer, third-party product developer or core developer.