Halliday & Matthiessen (2014: 232):
The ‘transformative’ type of ‘material’ clause covers a much wider range than the ‘creative’ type. As always, it is difficult to find an appropriate term for the grammatical category. We have to understand it in the context of the relevant systemic contrast. Thus ‘transformative’ means that the Actor (‘intransitive’) or Goal (‘transitive’) exists prior to the onset of the unfolding of the process, and is changed in some way or other through the unfolding of the process.