Halliday & Matthiessen (2004: 588, 588n):
The upper bound of the semantic stratum is the text: this is the most extensive unit of meaning. The upper bound of the lexicogrammatical system is the clause: this is the most extensive unit of wording. … By saying that they are the upper bounds, we are not ruling out complexes — text complexes and clause complexes … . But complexes are not higher–ranking units but rather expansions of units of a given rank.