I think you might mean that unreal is "source available" which doesn't confer any of the rights of open source, but does allow you to view and modify the source code subject to a commercial license (which might be free for personal, small scale use. I don't know unreal's pricing structure).
I think you might mean that unreal is "source available" which doesn't confer any of the rights of open source, but does allow you to view and modify the source code subject to a commercial license (which might be free for personal, small scale use. I don't know unreal's pricing structure).