Here is an exchange (1) from the TV show Friends:
Ross: [Entering] What is this dive? Only you could’ve picked this place.
Mrs. Bing: Oooh, c’mon, shut up, it’s fun. Gimme a hug.
At first this seems a simple exchange but I think for an EFL learner it actually could be quite difficult. It’s not entirely clear what is going on. It begins with the phrase, generally taught in the first lessons of EFL classes, ‘What is X?’ and so we would expect an answer of ‘It’s a Y.’ Yet from the exchange it is quite clear that they both know exactly what they are talking about. There is a chain of reference across the turns: this dive…this place…it. It seems then that it is not a simple request for information.
When we teach the structure ‘What is X?’ we are generally focused on some kind of unknown object or exchange of information, as in this example (2) from Friends:
Monica: What is it?
Rachel: Country club newsletter.
In this next example (3), however, the reference is known and the exchange is actually a request for an explanation:
ROSS: What is that? [referring to the sandwich]
JOEY: For the ride.
This results in an appropriate response, invoking Gricean conversation rules, of ‘For the ride‘ and not ‘It’s a sandwich‘. This difference between example 2 and 3 may not be entirely clear for an EFL learner.
Example 3 is, however, still essentially an exchange of information. In both example 2 and 3 the focus remains on the field of discourse which finds expression in ideational meanings and the choice of Participant in the Relational clause. The choice is between Country club newsletter (identifying intensive) or for the ride (attributive circumstantial). In example 1, however, there is no real clear link between the question (What is this dive?) and any apparent answer. Ross, in fact, seems to answer it himself (Only you could’ve picked this dive).
I think the focus here is actually on the tenor of discourse, the relationship between the speakers and evaluation of choice of venue. Mrs Bing (re)establishes the dominant power position between them with an imperative clause (shut up) yet the lexical choices (dive, picked, fun) indicate a close relationship between them. The choices at an expression stratum also reinforce this closeness (c’mon, gimme).
The choice of demonstrative, then, seems to reflect choices made at a register level and not really what they actually refer to.