Halliday & Matthiessen (2004: 558):
Like the personals and other demonstratives, the has a specifying function; it signals ‘you know which one(s) I mean’ … but … the merely anounces that the identity is specific; it does not specify it. The information is available elsewhere. It may be in the preceding text (anaphoric); in the following text (cataphoric); or in the air, so to speak … self-specifying; there is only one — or at least only one that makes sense in the context (homophoric).