Halliday & Matthiessen (2014: 333n):
Note that ‘transitivity’ is the name for the whole system, including both the ‘transitive’ model and the ‘ergative’ one. ‘Ergativity’ is thus not the name of a system, but of a property of the system of transitivity: within this system of transitivity, we can recognise the ‘transitive model’ and the ‘ergative model’
Halliday & Matthiessen (2014: 349):
A clause with no feature of ‘agency’ is neither active nor passive but middle. One with agency is non-middle, or effective, in agency. An effective clause is then either operative or receptive in voice. In an operative clause, the Subject is the Agent and the Process is realised by an active verbal group; in a receptive [clause] the Subject is Medium and the Process is realised by a passive verbal group.