Halliday & Matthiessen (1999: 220-1):
That those in (i) are being construed primarily as participants, whereas those in (ii) are being construed primarily as circumstances, is shown by a variety of other grammatical factors; to give just one example, the question equivalent to (i) is who? or what? (who did you give milk to?, not where did you give milk?), whereas the question equivalent to those in (ii) is how long?, when? (when are we meeting?, not what are we meeting on?). By the same token, in (iii) we have two different elements, one participant and one circumstance, but with hardly any difference between the two:
