Personal and demonstrative DEIXIS is a system of the nominal group, personal and demonstrative REFERENCE is not a system of the nominal group.
Personal and demonstrative DEIXIS serves an interpersonal function, personal and demonstrative REFERENCE serves a textual function.
Halliday & Matthiessen (2014: 367):
Many languages embody these two forms of deixis in the structure of the nominal group. The two are closely related, both being (as indicated by the term ‘deixis’) a form of orientation by reference to the speaker – or more accurately, to the ‘speaker-now’; the temporal-modal complex that constitutes the point of reference of the speech event.
Halliday & Matthiessen (2014: 623):
The textual status at issue in the system of reference is that of identifiability: does the speaker judge that a given element can be recovered or identified by the listener at the relevant point in the discourse or not? If it is presented as identifiable, then the listener will have to recover the identity from somewhere else. If it is presented as non-identifiable, then the listener will have to establish it as a new element of meaning in the interpretation of the text.
