And why are we using figures of being, rather than some other type of figure? Because ‘figures of doing and being can be interpreted as complementary perspectives on a quantum of change’ [Halliday & Matthiessen (1999: 132)], and here we are specifically interested in how phenomena are related to each other.
And figures of being, in general, involve not only the experiential relations of identity and ascription, but also the logical relations of expansion, namely: elaboration, extension & enhancement — the relations that also hold between figures in that other order of phenomena: the sequence.
And why are we using elaborating figures of identity, rather than extending or enhancing types ?
Because in this paper we are chiefly interested in the elaborative dimension of material and semiotic complexification, rather than, say, the extending relation of composition , or the enhancing relations of cause or condition, for example.