The SSL threat landscape has changed significantly since the rating guide was first published, back in 2009. The original text is thus no longer entirely adequate to deal with the threats of today. In addition, we now have several years of experience using the criteria in real life, and we know what works well and what not so much.
Our plan is to update the rating criteria to take all this new knowledge into account. However, because that process will take several months to complete and because an update to the guide is long overdue, we have decided to release a small patch release, labelling it 2009c. The year in the version number indicates that this is conceptually the same guide as previously published, with improvements.