Halliday & Matthiessen (1999: 453):
The second position is the reverse of the first in terms of the distribution of the nuclear process and participants. A relational process is interpreted on the model of a material one; the attribute (participant) is interpreted as if it was a nuclear process. Thus, to continue with the same artificial examples, Socrates is white is analysed as ‘Socrates whited’. The copula is not interpreted as representing a process but is thought to be only a “bearer” of tense, person and number.