Simple Thing: Substance

Halliday & Matthiessen (1999: 191):

substance

typifying role: thing as part of circumstance [Range in prepositional phrase, especially Location, e.g. on the ground], rather than having direct role as participant in figure.

pronoun it; general noun stuff; number category: mass.

has extension in space, but unbounded; can be manipulated and measured; if participant in figure, is typically being distributed [Goal, e.g, cut the string, keep rain out].