Halliday & Matthiessen (2014: 288):
Both ‘identifying’ and ‘attributive’ clauses of the ‘intensive’ kind have the option of assignment: they may be configured with a third participant representing the entity assigning the relationship of identity [or] attribution … In the case of ‘identifying’ clauses, this is the Assigner; in the case of ‘attributing’, this is the Attributor. In a ‘receptive’ clause, this participant may be left implicit …