Martin (1992: 23):
First, IDENTIFICATION. Consider the following proportionalities:
There was a robot : It looked bored ::
There was a robot : The android looked bored ::
There was a robot : This model looked bored
The relevant opposition here is between introducing a participant (a robot) and presuming one (it, the android, this model). In the examples, it, the android and this model assume that identity is recoverable from context, while a robot does not.
 Martin's IDENTIFICATION is Halliday's cohesive system of REFERENCE, reconstrued as 'discourse semantics'. Halliday & Matthiessen (2004: 550-1):
The textual status at issue in the system of reference is that of identifiability: does the speaker judge that a given element can be recovered or identified by the listener at the relevant point in the discourse or not? If it is presented as identifiable, then the listener will have to recover the identity from somewhere else. If it is presented as non-identifiable, then the listener will have to establish it as a new element of meaning in the interpretation of the text.
 The reference items in the examples are the grammatical items it, the and this which refer anaphorically to a robot. The lexical items android and model are related to robot by lexical cohesion (Martin's IDEATION), not by reference (Martin's IDENTIFICATION). This inconsistency will contribute to undermining the argument for this type of 'discourse structure'.