Halliday & Matthiessen (2004: 538):
… the systems of cohesion operate within either the grammatical zone or the lexical zone of the lexicogrammatical continuum. Conjunction, reference and ellipsis are all grammatical systems, and are thus all manifestations of what we might call grammatical cohesion. The point of origin of each of these systems falls within one or more particular grammatical unit; and terms within these systems are realised either by grammatical items that have some particular place within the structure of that unit or (in the case of ellipsis) by the absence of elements of grammatical structure. For example, the systemic environment of conjunction is that of the clause; and conjunctions serve as conjunctive Adjuncts in the structure of the clause. In contrast, lexical cohesion operates within the lexical zone; and it follows the general principle that lexical items are not defined in terms of particular grammatical environments.