Just happened over this Wikipedia article Locker-, Halbfest- und Festgestein – Wikipedia - it is in German only, but it is very clear that clay belongs to the loose set, like pebblestone, shingle, scree, gravel, fine gravel, sand &c. It is just composed of the finest particles in the set. No idea how to translate that into “firmness®”, which is a registered trademark in OSM, from what I learned, even though nobody really able to tell, what it is about.