Halliday & Matthiessen (2014: 312):
But if we think of ‘circumstantiation’ as a general concept, in the context of the overall interpretation of transitivity as the grammar of experience, we can get a sense of the semantic space that is being constructed by these circumstantial elements. One way of doing this is to relate them to the various types of process that have been described above.
We are able to do this because a circumstantial element is itself, from this point of view, a process that has become parasitic on another process. Instead of standing on its own, it serves as an expansion of something else. Most circumstantials can be derived from the three types of relational process [i.e. intensive, possessive, circumstantial]; the largest group, not surprisingly, from that type of relational process for which we use the label ‘circumstantial’.