Halliday & Matthiessen (2014: 225-6):
[The Actor] brings about the unfolding of the process through time, leading to an outcome that is different from the initial phase of the unfolding. This outcome may be confined to the Actor itself, in which case there is only one participant inherent in the process. Such a ‘material’ clause represents a happening and, using traditional terminology, we can call it intransitive. Alternatively, the unfolding of the process may extend to another participant, the Goal, impacting it in some way: the outcome is registered on the Goal in the first instance, rather than on the Actor. Such a ‘material’ clause represents a doing and we can call it transitive.