Halliday & Matthiessen (2014: 228, 230):
The nature of the outcome affecting the Actor of an ‘intransitive’ clause and the Goal of a ‘transitive’ one [i.e. the Medium] turns out to be the general criterion for recognising more delicate subtypes of material clauses. The most general contrast is between
(i) ‘creative’ clauses, where the Actor or Goal is construed as being brought into existence as the process unfolds, and
(ii) ‘transformative’ ones, where a pre-existing Actor or Goal is construed as being transformed as the process unfolds.