Halliday & Matthiessen (2014: 370-1):
In contrast, when a nominal group refers to a class, the use of determiners as Deictic is more constrained. For example, while this dying elephant and this elephant are both possible as anaphoric references to a particular member of the class of elephant, only the elephant would normally be possible as a reference to the general class of elephants. In addition, certain options that are quite distinct as references to members of a class are much closer in meaning as references to the class itself; these are marked ,  and  in Table 6-7.
Note that Matthiessen here again follows Martin (1992) in blurring the distinction between reference in the sense of ideational denotation and reference in the sense of textual cohesion. Nominal groups can denote classes or their members (ideational metafunction); in anaphoric reference, a reference item refers to a referent, creating cohesion (textual metafunction).