There was a schism in the RSS 0.x days. 2.0 is Really Simple Syndication and has a looser approach; RSS 1.0 is RDF Site Summary and tightens things up. Really,

There was a schism in the RSS 0.x days. 2.0 is Really Simple Syndication and has a looser approach; RSS 1.0 is RDF Site Summary and tightens things up. Really, if you’re going to generate feeds, you should use Atom or JSON Feed these days. Too much baggage with RSS that makes it difficult to parse unambiguously.