Halliday & Matthiessen (1999: 212):
Qualities of extension resemble those of ‘elaboration: identity’ in that they construe a relation between the thing they are assigned to and some other thing. The relation typically obtains between the thing as a discoursal instance rather than the thing as a general experiential class. Thus an alternative solution is a solution that can replace the one we have just been talking about. Consequently, properties of extension tend to serve as deictic elements (Post-Deictic) rather than as Epithets; they indicate how an instance (or instances) of the general class of the thing they are assigned to is selected from that class: we need an additional two volunteers means ‘two further instances of the general class of volunteer’. Like other properties serving as Post-Deictic, they may precede the Numerative (additional^two); but they may also follow, with no strong contrast in meaning: we need two additional volunteers.