Halliday & Matthiessen (2014: 664, 665):
The account that we have outlined is summarised in Table 10-1. The table shows the intersection of two content strata (semantics and lexicogrammar) with the three metafunctions. Within each metafunction there are semantic patterns that are intermediate between (1) the local semantic units that are realised by the clause and the clause complex and (2) the global semantic unit, i.e. the text.
Table 10-1 shows how semantic units are mapped onto grammatical ones. The principle is that of rank-based constituency – semantic unit a ↘ grammatical unit m; the key grammatical unit is the clause, as shown diagrammatically in Figure 10-1. But while this is the foundation on which the relationship between semantics and lexicogrammar is based, there are two other principles affecting this relationship, making it more complicated but also extending the meaning potential of language: (i) transgrammatical semantic domains – domains of meaning extending across different grammatical units, and (ii) metaphor – incongruent realisational relations between semantics and lexicogrammar.