Halliday & Matthiessen (1999: 106):
This differentiation is embodied in relations of sequence in the following way. Either a sequential relation expands one figure by adding another one to it, the two still remaining at the same phenomenal level; or the sequential relation projects one of the two figures onto the plane of second-older, semiotic phenomena, so that it enters the realm of metaphenomena (meanings or wordings). This is the distinction between expansion and projection. For example,
expansion: highs will be mid-8Os to mid-90s —> but parts of Texas could reach the 100s;
projection: the forecast predicts —> "parts of Texas could reach the 100s".