Halliday & Matthiessen (2014: 558):
Group and phrase complexes thus serve to develop single elements within a clause (or, if these complexes are embedded, a single element within a group or phrase), serving the same function as a simple group or phrase would. Textually, this means there is a single message; interpersonally, it means there is a single proposition or proposal; and experientially, it means that there is a single figure.