REFERENCE. A participant or circumstantial element introduced at one place in the text can be taken as a reference point for something that follows. In the simplest case this means that the same thing comes in again, like 'the boy who looks after the sheep . . . he . . . him . . . he' above. But it may also mean that it serves as a basis for comparison, like 'Henry . . . someone else' in 'Henry can’t play today. We’ll have to find someone else', where 'someone else' means ʻsomeone other than Henryʼ.
- Halliday (1985), Pag. 288

Reference is a relationship between things, or facts; it may be established at varying distances, and although it usually serves to relate single elements that have a function within the clause (processes, participants, circumstances), it can give to any passage of text the status of a fact, and so turn it into a clause participant.
- Halliday (1985), Pag. 289

Reference is a relationship in meaning. When a reference item is used anaphorically, it sets up a semantic relationship with something in the preceding text; and this enables the reference item to be interpreted, as either identical with the referent or in some way contrasting with it.
- Halliday (1985), Pag. 295-296