Halliday & Matthiessen (2014: 84):
The general principle of discreteness means that each structural unit has clearly defined boundaries. But while this kind of segmental organisation is characteristic of the clause as representation, the clause in its other guises – as message, and as exchange – departs from this prototype. In its status as an exchange, the clause depends on prosodic features — continuous forms of expression, often with indeterminate boundaries; while in its status as message it tends to favour culminative patterns — peaks of prominence located at beginnings and endings.