Halliday & Matthiessen (1999: 175-6):
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 receiver or recipient in the process. Thus this general motif is manifested in a form which corresponds to its ecological niche at that location. Note that the boundaries do not exactly coincide across the different bands of the helix; in any case, they are fuzzy, and they tend to become more fuzzy with increasing distance from the centre. Our characterisation here is inevitably somewhat overdeterminate. The outer circumstances, in turn, are typically agnate to clauses (e.g. while he was in LA, he sent a parcel); thus we could construe the same general relationships over again in the form of a sequence — grammatically, as one nexus in a clause complex.