'Deictic'. The Deictic element indicates whether or not some specific subset of the Thing is intended; and if so which. It is either (i) specific or (ii) non-specific […] The subset in question [specific] is specified by one of the two possible DEICTIC features: either (i) demonstratively, i.e. by reference to some kind of PROXIMITY to the speaker […] or (ii) by possession, i.e. by reference to PERSON as defined from the standpoint of the speaker […] together with the possibility of an interrogative in both these categories […] All these have the function of identifying a particular subset of the ʻthingʼ that is being referred to.
- Halliday (1985), Pag. 160

There are a number of elements in language that are inherently ʻgivenʼ in the sense that they are not interpretable except by reference to some previous mention or some feature of the situation [...] deictic elements (those that are interpreted by reference to the ʻhere-&-nowʼ of the discourse). Typically these items do not carry information focus; if they do, they are contrastive. So when we say that, for any information unit, the unmarked structure is that with the focus on the final element, this excludes any items that are inherently given.
- Halliday (1985), Pag. 277