Halliday & Matthiessen (2004: 550):
But whereas Theme and New are parts of textual structures — Theme ^ Rheme in the clause and Given + New in the information unit respectively, the textual statuses that come under the heading of cohesion, reference and ellipsis, are not. That is, while an element is marked cohesively as identifiable by means of a grammatical item such as the personal pronoun they, or as continuous by means of a grammatical item such as the nominal substitute one, the textual statuses of identifiability and continuity are not structural functions of the clause or of any other grammatical unit. They can occur freely within Theme or Rheme, and within Given and New (although there are certain unmarked associations).