[fao_]

I think it refers to the type of break the joint makes?

However, while searching I found this search excerpt:

    Which metals diffuse into other metals and how? | Naked ...
    www.thenakedscientists.com › forum
        Sodium and potassium metal (both *soft solids, like cold butter*) react

So perhaps it is used to designate 'soft solids' versus 'hard solids'