Halliday & Matthiessen (1999: 13):
Since our approach is via the grammar, we have taken the boundaries of the grammar as criterial, using the clause complex — the highest rank of ideational organisation —to define the scope of the ideational-semantic representation. This is not a necessary constraint; but it is one that is motivated in terms of the overall design, and which may turn out to define the optimal moment of interfacing between the ideational and the other components. … The constraint does not imply, however, that the scope of ideational semantics does not extend over sequences longer than a clause complex.