Martin (1992: 271):
To this point, the point of departure for exploring discourse systems has been a grammatical one: general MOOD classes (realising NEGOTIATION), closed system nominals (tracking participants) and clause complex structures (used extensively to code conjunctive relations in spontaneous spoken monologue). In this chapter on the other hand the point of departure is lexis. It is the contribution made by open system items to discourse structure that is under consideration. This is an ambitious undertaking, in at least two respects. First, lexis has received less attention in functional linguistics than has grammar, and so there is less to build on… . And second, the scope of experiential meaning coded through lexis in any language is vast, which fact alone makes it harder to bring under analytic control. Nevertheless, lexical relations have an important role to play in discourse structure.
 This misrepresents what was done in chapter 4. Non-trivially, the chapter applied Halliday's logical semantic relation of expansion — not clause complex structures — without distinguishing whether it was deployed structurally in logical complexes, or non-structurally to cohesively mark textual transitions. Trivially, the model was applied to written as well as spoken mode, and to dialogue as well as monologue.
 It is true that the systems of sufficient delicacy to account for lexis have not been developed in SFL theory. As Halliday and Matthiessen (2004: 46) have estimated:
It would take at least 100 volumes of the present size to extend the description of the grammar up to that point [of maximum delicacy] for any substantial portion of the vocabulary.
On the other hand, SFL theory models the discourse function of lexis as lexical cohesion, in which the types of lexical relations include those of repetition, synonymy, hyponymy, meronymy and collocation.
 In SFL theory, the experiential meaning "encoded through lexis" is brought "under analytic control" by
- construing meaning as a higher level of abstraction than the lexicogrammar, and
- modelling lexis as most delicate grammar, such that grammatical systems represent the most general of the systems whose ultimate specification is lexical items.