Halliday & Matthiessen (1999: 191-2):
abstraction (material)typifying role: as Phenomenon in figure of sensing [e.g. estimate the depth], as participant in figure of being [Carrier in ascriptive figure, e.g. the colours were too bright; Value in identifying figure, e.g. the score was 2-1].pronoun it; general noun: none; number category: mass.has no extension in space and is unbounded; typically some parameter of a material quality or process.