Halliday & Matthiessen (1999: 218-9):
We referred to the cline from participants to circumstances as the “degree of involvement” in the actualisation of the process. This degree of involvement ranged from the closest, the Medium, which is part of the nucleus of the figure, to those that appear most remote, circumstances such as Matter (e.g. concerning your request) and Angle (e.g. in my own opinion). Somewhere in the middle was an area of overlap, where participants and circumstances are very closely akin. The two overlap because there are some functions which can be construed either as a form of participant in the process or as a circumstance attendant on it.