I once took a class in Chinese history. For the final examination we needed to learn terms and be able to identify these terms. One such term was “the fengjian system”. I looked it up in the course textbook and the only thing written about the fengjian system was that it was not feudalism.
From that I concluded it probably had to be a form of feudalism.
Because there are a lot of things that are not feudalism. I’m not feudalism. Presumably you’re not feudalism. But only if something is pretty much feudalism, is it worth it for the writer of a textbook to define it as “not feudalism”.