Halliday & Matthiessen (2014: 221):
One way of looking at the situation is this. The process is the most central element in the configuration. Participants are close to the centre; they are directly involved in the process, bringing about its occurrence or being affected by it in some way … and we can say that the configuration of process + participants constitutes the experiential centre of the clause. Circumstantial elements augment this centre in some way — temporally, spatially, causally and so on; but their status in the configuration is more peripheral and unlike participants they are not directly involved in the process.