We can now see more clearly what the terms Given and New actually mean. 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.
- Halliday (1985), a pag.277

There is a close semantic relationship between information structure and thematic structure [...] Other things being equal, a speaker will choose the Theme from within what is Given and locate the focus, the climax of the New, somewhere within the Rheme. But although they are related, Given + New and Theme + Rheme are not the same thing. The Theme is what 'I', the speaker, choose to take as my point of departure. The Given is what you, the listener, already know about or have accessible to you. Theme + Rheme is speaker-oriented, while Given + New is listener-oriented.
- Halliday (1985), a pag.278