Halliday & Matthiessen (1999: 191):
object (material)typifying role: impacted participant in figure of doing [Goal, e.g. build a house, pick up sticks].pronoun it/ them; general noun thing; number category: count (singular/ plural).has extension in space, bounded so participates in figures as unit whole; if acting, then in figure of happening [Actor in involuntary process, e.g. the button fell off}.