Halliday & Matthiessen (1999: 13):
[This] does not imply, however, that the scope of ideational semantics does not extend over sequences longer than a clause complex.
Halliday & Matthiessen (1999: 528):
Since these resources [of the textual metafunction] are oriented towards discourse, many of the “textual” systems in any language have a domain potentially higher than the clause and the clause complex; they set up relationships that create semantic cohesion, and these are not restricted by the limitations of grammatical structure.
Halliday & Matthiessen (1999: 530):
… the “textual” metafunctional component comprises a further set of resources, which construe clauses and clause complexes into longer stretches of discourse without the formality of further grammatical structure. … These are of four kinds: reference …, ellipsis, conjunction and lexical cohesion.