Halliday & Matthiessen (2014: 84):
So far, we have referred to constituent structure as if it was something uniform and homogeneous; but as we embark on the detailed analysis of clause structures this picture will need to be modified. The model of constituent structure that we presented — the rank scale — is the prototype to which all three metafunctions can be referred. But the actual forms of structural organisation depart from this prototype, each of them in different ways.