Halliday & Matthiessen (2004: 550-1):
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.