Halliday & Matthiessen (2014: 49):
Structure is analysed in functional terms, explaining the part played by each element in the organic configuration of the whole. We shall see later on that the configurational view of structure is oversimplified, if not distorted, because the way linguistic units are structured tends to vary according to metafunction. But it is possible to reduce all types of structure to a configurational form, as a strategy for exploring the grammar.