To interpret relations between a technical field and its realisation as text, the challenge for linguists, as ever, is to escape the limits of grammar. Fields are realised as patterns of ideation - taxonomic relations, nuclear relations and activity sequences, organised in hierarchies of periodicity.
 This blurs the distinction between stratification and instantiation. In terms of stratification, field, the ideational dimension of context, is realised in semantics. In terms of instantiation, text is an instance of potential. As an instance of potential, a text realises an instance of context, namely: a context of situation.
 This is merely an unsolicited plug for Martin's discourse semantics. (For a very thorough critique of this theory, see here.) The misunderstanding here is stratificational. Grammar and semantics are two levels of symbolic abstraction, two angles on the same phenomenon, namely: the content plane of language. As Halliday points out, the semantics that language has is only made possible by the grammar. The grammar doesn't just realise the semantics, it construes it.
 This again blurs the distinction between stratification (realisation) and instantiation (patterns). Field is realised by semantics (stratification), system is realised by structure (axis), on each stratum. Patterns are formed by the selection of features during the instantiation process, whereby systemic potential is actualised as text during logogenesis.
 As the critiques on Discourse Semantic Theory demonstrate, Martin's experiential discourse semantic system of ideation is a confusion of lexical cohesion (textual metafunction), lexis as most delicate grammar (delicacy) and logical relations between elements of clause structure (mostly misapplied). Of his model, Martin (1992: 325) writes:
The level of discourse semantics is the least differentiated as far as ideational meaning is concerned. This is mainly due to the fact that the description developed here has focussed on relationships between experiential meanings, rather than the experiential meanings themselves.