Halliday & Matthiessen (2014: 669-73, 676):
Let us now present a systematic and comprehensive summary of the different grammatical environments in which elaboration, extension and enhancement are manifested: see Table 10-3. As the table shows, the environments of manifestation can be differentiated in terms of (i) metafunction – textual (CONJUNCTION), logical (INTERDEPENDENCY; MODIFICATION) and experiential (CIRCUMSTANTIATION; PROCESS TYPE: relational), and (ii) rank – clause and group/phrase. (The table could, in fact, be extended downwards along the rank scale to take account of patterns below the rank of group/phrase within the logical metafunction: word and morpheme complexes also embody interdependency relations that combine with expansion.)
From a grammatical point of view, the environments set out in Table 10-3 are, of course, all different. But seen from above, from the vantage point of semantics, they are all agnate ways of construing expansion. Collectively they thus construe expansion as a semantic system. This means that 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.
… Let’s now turn to the semantic domain of projection. Like expansion, projection is manifested both logically and experientially within the ideational metafunction; but outside the ideational domain, it is manifested interpersonally rather than textually, thus contrasting with the textual manifestation of expansion: see Table 10-5.