Halliday & Matthiessen (1999: 520):
One is that of “expansion”, in which the two processes are of the same order of experience and the second one is interpreted as in some respect expanding on the first. The other is that of “projection”, in which the second process is construed as belonging to a different order of experience: it is projected, by the first one, on to the semiotic plane. Each of these defines a complex region of semantic space.