Halliday & Matthiessen (1999: 188):
There is thus a marked contrast between things and all other elements. A quality enters into a figure only as an Attribute; a process only as a Process; and a circumstance only in some particular circumstantial rôle. Other than this, the only functional environment for qualities, processes and circumstances is that where they form parts of things — that is, grammatically, where they enter into the structure of the nominal group. The fact that these other elements can themselves enter into the specification of a thing is another indication of the relative complexity of things.