Halliday & Matthiessen (2014: 348):
These, seen from a transitive perspective, are circumstantial: Agent is a kind of Manner, Beneficiary is a kind of Cause and Range is a kind of Extent; and they can all be expressed as minor processes. But seen from an ergative point of view they are additional participants in the major process: the nucleus of ‘Process + Medium’ has an inner ring of additional participants as well as an outer ring of circumstances surrounding it …