Halliday & Matthiessen (2014: 29-30):
As grammarians we have to be able to shift our perspective, observing now from the system standpoint and now from that of the text; and we have to be aware at which point we are standing at any time. … Writing a description of a grammar entails constant shunting between the perspective of the system and the perspective of the instance.
Note that speaking doesn't involve shunting up and down the cline of instantiation. Martin's notion of "distantiation" or "de-instantiaton" arises from this misunderstanding.