Halliday And Matthiessen (2014: 669, 673):
… for any given type of expansion we want to express, we have at our disposal a range of resources. For example, if we want to express an enhancing relationship of cause, the opportunities made available by the grammar include the one set out in Table 10-4.
These patterns of wording are agnate, but just like agnate patterns in general they are not synonymous: agnation always embodies both similarity and difference. The similarity is the basis for interpreting the patterns as alike, bringing them together within a paradigm, while the difference is the basis for treating them as variant types rather than as tokens of the same type. The patterns are similar in that they are all manifestations of different kinds of expansion, as illustrated by the examples of the enhancing notion of cause above. But how do the agnate variants differ in meaning? As always with questions of meaning, the answer can be found in its metafunctional organisation: differences turn out to be (i) ideational, (ii) textual and (iii) interpersonal.