Martin (1992: 499):
Instead, taking Halliday's (1978) model [of context] as a baseline, the problem of purpose will be explored, since it bears critically on the relationship between genre and register to be further developed below.
 In SFL theory, the "purpose" of a functional variety of language is modelled in terms of the rhetorical mode of the situation type that the language realises. Mode is concerned with the rôle that language plays.
 In SFL theory, register and genre — in the sense of text type — are the same phenomenon viewed from different poles of the cline of instantiation. Register is the view of functional varieties of language from the system pole; text type is the view from the instance pole. Importantly, these are identified by the contextual features — field, tenor and mode — that they realise. That is, situation type (context) and register/text type (language) are different levels of symbolic abstraction.