Halliday & Matthiessen (2014: 628-9):
Personal reference items create co-reference in terms of the category of person. As we noted above, there is another related, but distinct, co-referential strategy – that of demonstrative reference. Here the reference item is a demonstrative, this/that, these/those. Demonstratives (see Table 9-11) may also be either exophoric or anaphoric; in origin they were probably the same as third-person forms, but they retain a stronger deictic flavour than the personals, and have evolved certain distinct anaphoric functions of their own.
