Halliday & Matthiessen (2014: 434):
… a circumstantial element in a clause contains only a minor process, not a major one; so unlike a clause it cannot construe a figure, it cannot enact a proposition/proposal and it cannot present a message. In contrast, clause complexing always involves assigning clause-hood to an augmentation of expansion or projection: the augmentation has the full potential of a clause, in terms of experiential, interpersonal and textual systems. … while circumstantial elements are part of the ‘configurational’ organisation of the clause, clauses in clause complexes are part of a chain-like or serial structure.