Halliday & Matthiessen (2014: 118):
The significant variable is: information that is presented by the speaker as recoverable (Given) or not recoverable (New) to the listener. What is treated as recoverable may be so because it has been mentioned before; but that is not the only possibility. It may be something that is in the situation, like I and you; or in the air, so to speak; or something that is not around at all but that the speaker wants to present as Given for rhetorical purposes. The meaning is: this is not news. Likewise, what is treated as non-recoverable may be something that has not been mentioned; but it may be something unexpected, whether previously mentioned or not. The meaning is: attend to this; this is news. One form of ‘newness’ that is frequent in dialogue is contrastive emphasis.