Halliday & Matthiessen (2004: 563):
Ellipsis marks the textual status of continuous information within a certain grammatical structure. At the same time, the non-ellipsed elements of that structure are given the status of being contrastive in the environment of continuous information. Ellipsis thus assigns differential prominence to the elements of a structure: if they are non-prominent (continuous), they are ellipsed; if they are prominent (contrastive), they are present. The absence of elements through ellipsis is an iconic realisation of lack of prominence.