Halliday & Matthiessen (1999: 192-3):
abstraction (semiotic)typifying role: scope-defining participant in figure of sensing or saying [Range in mental process, e.g. find out more information; in verbal process, e.g. tell the truth]; also participant in figures of being & having [e.g. possessed Attribute, have you any evidence].pronoun it; general noun: some attitudinally loaded ones such as nonsense [non-attitudinal ones for some, e.g. idea, fact]; number category: mass.unbounded semiotic substance; may be qualified by projection [as Thing + Qualifier in nominal group, e.g. the knowledge that they had failed]', no material existence.