Halliday & Matthiessen (2014: 280):
It is this directionality that determines the voice of the clause – whether it is ‘operative’ or ‘receptive’; and in order to explain this we need to operate with Token and Value as structural functions. … In other words, ‘identifying’ clauses select for voice; they have an ‘operative’ and a ‘receptive’ variant. The difference is entirely systematic, once we recognise the structure of Token and Value: the ‘operative’ voice is the one in which the Subject is also the Token (just as, in a ‘material’ clause, the ‘operative’ is the variant in which the Subject is also the Actor).