Halliday & Matthiessen (1999: 63):
We can recognise two simultaneous distinctions. One concerns the type of circumstantial relation construed; the primary contrast is between circumstances of projection and circumstances of expansion and within the latter we distinguish those of elaboration, extension and enhancement. The other concerns the experiential complexity of the circumstance; circumstances are either ‘simple’ or ‘macro’, the former being truly elemental while the latter are more like figures. … Among simple circumstances, the most usual are those of time, place, manner-quality and intensity, all of which are circumstances of enhancement. … Macro circumstances are those which are made up of a special type of figure having another participant inside it …