Martin (1992: 271-2):
Something of the role played by lexical relations is illustrated in [5:1] below. A mother is talking with her 10 year old son about his experiences at school earlier in the day. Towards the end the mother realises that she may have misinterpreted the field, and interrupts to clarify the situation. It is the lexical cohesion that has been misread, not the NEGOTIATION, IDENTIFICATION or CONJUNCTION. …
Meanwhile it suffices to comment that it is the relations between lexical items, not lexical items per se that have been misunderstood. And the misreading of these relations is the misreading of the text's field. … This means that although considerations of register have been set aside as far as possible in Chapters 2, 3 and 4, with lexical cohesion the register variable field needs to be brought more clearly into the picture.
 In the 33-line text, the child begins talking about choosing teams in kickball then switches to playing baseball, upon which the mother realises she had mistaken kickball for soccer. The first field is identified in the second clause:
In s…kickball we have two captains, right?
and second field is identified in the 25th clause:
I got a home run today.
It is not the lexical cohesion that has been misunderstood — not the lexical relations of repetition, synonymy, hyponymy, meronymy and collocation. The mother misread the first field because she misheard 's…kickball' as 'soccer'. It was the experiential meaning that was misunderstood, not the textual cohesion.
 Given what is to come, it is worth mentioning that this use of situation and field is consistent with SFL theory. The situation is the instance of the context that is realised by the text, and the field is the ideational dimension of context. Note that this is second order field — what the language is about —which Martin later (op cit: 292) dismisses as less 'appropriate' than his conception of first order field.
 This misconstrues context as register. To use Martin's words (p260): 'this short-coming is a very serious one indeed'. In SFL theory, field is not a register variable, field is a dimension of context, and context is more symbolically abstract than language. Register, on the other hand, is not more abstract than language; register is a text type: a functional variety of language (not of context) that varies with the situation type (context) being realised.
The relation between language and context is realisation: an identifying relational process. The relation between language as system (potential) and language as register (subpotential) is instantiation: an ascriptive relational process.
The relation between context (situation type) and register (text type) is realisation. The lower order of symbolic abstraction, text type (language), realises the higher order, situation type (context).