Halliday & Matthiessen (2014: 365-7):
The specific Deictics … are demonstrative or possessive determiners, or embedded possessive nominal groups. The sub-set in question is specified by one of two possible deictic features: either (i) demonstratively, that is, by reference to some kind of proximity to the speaker … or (ii) by possession, that is, 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 ‘thing’ that is being referred to.