Halliday & Matthiessen (1999: 401):
These two clause systems, THEME IDENTIFICATION and THEME PREDICATION, are components of the overall system of THEME in English grammar. They produce structures in which identifying relational processes are used to reconstrue figures as equations (the two together are referred to as "thematic equatives" in Halliday, 1985: 41-4). But ideational grammatical metaphors typically have a discourse function of this kind; they are as it were pressed into service by the textual metafunction, to provide alternative groupings of quanta of information.