Martin (1992: 23):
Univariate structure at the rank of clause brings us to a grammatical frontier — the distinction between the clause complex and cohesion discussed in Section 1.4 above. As noted, Halliday and Hasan refer to this frontier as non-structural; and from the perspective of lexicogrammar, this is just what they are — semantic relationships which transcend grammatical structure.
 The 'distinction between the clause complex and cohesion' — 'from the perspective of lexicogrammar' — is the distinction between the structural resources of logical metafunction and the non-structural resources of the textual metafunction.
 Trivial point: it is not the "frontier" that is non-structural; but cohesion.
 Cohesive relations are also non-structural from the perspective of discourse semantics, as will be demonstrated in later posts.
 Cohesion realises 'semantic relationships which transcend grammatical structure'. Cohesion is a grammatical system — a textual system at the level of wording — and as such, realises textual systems at the level of meaning (semantics). Cohesion is a system at a lower level of symbolic abstraction than semantics.