Halliday & Matthiessen (2014: 216):
There is no priority of one kind of process over another. But they are ordered; and what is important is that, in our concrete visual metaphor, they form a circle and not a line. (More accurately still … a sphere … .) That is to say, our model of experience, as interpreted through the grammatical system of transitivity, is one of regions within a continuous space; but the continuity is not between two poles, it is round in a loop.