Halliday & Matthiessen (2014: 84):
The general principle of hierarchy means that an element of any given rank is constructed out of elements of the rank next below. This is a feature of the constituent hierarchy made up of units and their classes: clause, verbal group, and so on. But the configurations of structural functions show further ramifications of this general pattern. Thus, in the clause as exchange there is slightly more layering in the structure, while in the clause as message there is rather less.