Martin (1992: 504-5):
For Hasan, text structure is the realisation of choices made from among the options constituting a culture's field, mode and tenor (each permissable [sic] combination of choices is referred to by Hasan as a contextual configuration). In practice, obligatory elements of structure appear to derive from field, with variations in generic structure controlled by tenor and mode. This means that there is a strong association between field, text structure and genre. As Hasan (1985/1989: 61) puts it:
by implication, the obligatory elements define the genre to which a text belongs; and the appearance of all these elements in a specific order corresponds to our perception of whether the text is complete or incomplete.
 This misrepresents Hasan (1985: 59-63) in a way that encourages a misreading of the provided quote, and in a way that suits Martin's proposed model of text structure. In describing a particular service encounter, Hasan (1985: 60) writes of the first obligatory element, SALES REQUEST: 'Its occurrence is predicted mainly because of field values' and of the second, SALES COMPLIANCE: 'The motivation for SC is to be found in field and tenor values'. Hasan associates none of the other three obligatory elements (SALE, PURCHASE and PURCHASE CLOSURE) with a specific contextual value.
 This also misrepresents Hasan (1985: 59-63) in a way that encourages a misreading of the provided quote, and in a way that suits Martin's proposed model of text structure. Hasan (1985: 60) relates optional elements to contextual configurations (field, tenor and mode), not merely to tenor and mode:
So, while optional elements do not occur randomly, their optionality arises from the fact that their occurrence is predicted by some attribute of a CC that is non-defining for the CC and to the text type embedded in that CC.
 Given the false representations of Hasan above, the singling out of field here is misleading. On Hasan's model, the structure of a text of a particular type (genre), realises the field, tenor, and mode values that make up a contextual configuration.