Halliday & Matthiessen (1999: 146):
Elaboration and extension are agnate, one with the other; they offer alternative modes of construal, often with little apparent difference. … But if we technicalise these alternatives, they do constitute significantly different approaches to the interpretation of meaning … in systemic-functional work, elaborating interpretations tend to be taken further than in many other approaches: this means emphasising realisation, delicacy and identities across metafunctions to supplement the traditional emphasis on constituency and composition.