Halliday & Matthiessen (1999: 173):
… cutting across this cline of involvement, we find that — like the participants themselves — the circumstantial elements fall into distinct types according to their relationship to the Process + Medium nucleus. These types correspond to the four transphenomenal categories of the logico-semantic relations that are now familiar: the circumstance is either a circumstance of projection or a circumstance of expansion and, if the latter, then either elaborating, extending or enhancing.