Halliday & Matthiessen (1999: 171-2):
We have summarised the features of two participant rôles, Goal and Range, which vary considerably in their degree of participanthood, lying as it were at two ends of this continuum. We saw earlier that the Medium is the element that is most closely bonded to the Process, the two together forming the nucleus of the figure. Thus the highest degree of participanthood is that of whichever element, in each particular type of figure, is conflated with the generalised function of Medium; in the case of a figure of doing, this is the Goal, the element that is impacted (moved, changed, created or destroyed) by the Process.
At the other end of the cline are those elements whose status as participant is highly precarious, those which conflate with the generalised function of Range. These, as we have seen, are closely agnate to other types of figure, either those consisting of Process alone or those with Process + circumstantial element. We can thus extend the continuum further, outside the status of participant altogether, into the realm of circumstances […] circumstantial rôles and […] their degree of involvement in the process.