Halliday & Matthiessen (2004: 644):
… a figure, realised metaphorically by a nominal group rather than congruently as a clause, gains access to the textual systems of the nominal group — most significantly, the system of determination. This means it can be treated textually as a discourse referent. It is marked either as ‘non-specific’ or as ‘specific’, in which case the identity is presented as recoverable to the addressee.