Halliday & Matthiessen (2004: 601):
… this move involves a shift in metafunction … [and] the meaning of expansion changes with the change of metafunctional manifestation. For example, the manifestation of cause changes from rhetorical relation (textual: consequently) via logico-semantic relation (logical: so, because) to process or minor process or even participant (experiential: cause, through; cause). This means that the category meaning of ‘cause’ changes …