Halliday & Matthiessen (1999: 175):
What this brings out is that there are a small number of very general domains within this overall semantic space, which may be construed in different ways according to the status they are assigned within the figure. For example, there is one area that is concerned with the spatial orientation of the process. Construed as an outer circumstance, this appears as the position within which the process unfolds; construed as an inner circumstance, it means the direction towards which the process is oriented; construed as a participant, it shows up as the receiver or recipient of the process. Thus this general motif is manifested in a form which corresponds to its ecological niche at that location.