Halliday & Matthiessen (1999: 192):
typifying rôle: scope-defining participant in figure of saying [Range in verbal process, e.g. read the notice, tell me a story]; also active participant [Sayer, e.g. the book says, the regulations require …]
pronoun it/them, they; general noun (none); number category: count (singular/plural).
may also exist as a material object, e.g. book, clock; has the potential for being a symbol source [hence Sayer in projecting clause].