Halliday & Matthiessen (2004: 587):
The patterns that are developed in this way are, however, patterns of meaning, not patterns of wording; they are patterns at the level of semantics rather than the level of lexicogrammar. This is so because text is, as we have emphasised, a semantic phenomenon in the first instance; it is meaning unfolding in some particular context of situation. For example, the grammatical system of conjunction gives speakers and writers the resources to mark transitions in the development of a text — that is, to mark rhetorical relations used to expand the text step by step; and the rhetorical relations that are marked in this way are semantic relations organising the text as a flow of meaning.