Halliday & Matthiessen (2014: 716):
To sum up: when a sequence is realised by a clause rather than by a clause nexus, it will be structured textually into Theme + Rheme and, since a clause is an information unit in the unmarked case, also into Given + New. This means that the figures that make up the sequence can be given thematic or newsworthy status. In addition, such a figure, realised metaphorically by a nominal group rather than congruently by a clause, gains access to the textual systems of the nominal group – most significantly, the system of DETERMINATION. This means that it can be treated textually as a discourse referent. It is marked either as ‘non-specific’ or as ‘specific’, in which case its identity is presented as recoverable to the addressee.