Halliday & Matthiessen (1999: 203):
Things of this category are the names of types of projection; in the taxonomy of things, they are semiotic abstractions, some discrete and some non-discrete, and as a grammatical class they are referred to under the heading of fact nouns. They are grammatically distinct because they can function as Thing in a nominal group with a fact clause as Qualifier … Things of this type have an important rôle in discourse, because they function anaphorically to refer to (and at the same time classify) previous sections of text interpreted as projection.