Halliday & Matthiessen (2014: 623n):
Note that the term ‘reference’ has been used in different ways. For example, in philosophical and formal semantic works on meaning it indicates ideational denotation, as when expressions are said to refer to phenomena. (In such contexts, reference (or extension) and sense (or intension) are often taken as complementary aspects of meaning, going back to Frege’s distinction between Sinn and Bedeutung.) Here we are using the term in the way it has been used in functional work (e.g. Halliday & Hasan, 1976) to indicate the textual cohesive strategy discussed in this section.