Halliday & Matthiessen (2004: 550n):
Note that the term ‘reference’ has been used in different ways. For example, in philosophical works on meaning it indicates ideational denotation, as when expressions are said to refer to phenomena. Here we are using the term in the way it has been used in functional work (eg Halliday and Hasan 1976) to indicate the textual cohesive strategy …