Halliday & Matthiessen (1999: 29):
In the interpersonal and textual domains of content, it is often natural; thus interpersonal content tends to be represented prosodically by movements or variant levels in pitch, and textual content tends to be represented by prominence achieved phonologically (e.g. by the major pitch movement in an intonation contour) or sequentially (e.g. by using distance from initial position in the clause as a scale of prominence).