Halliday & Matthiessen (2014: 118):
There are a number of elements in language that are inherently ‘given’ in the sense that they are not interpretable except by reference to some previous mention or some feature of the situation: anaphoric elements (those that refer to things mentioned before) and deictic elements (those that are interpreted by reference to the ‘here-&-now’ of the discourse). Typically these items do not carry information focus; if they do, they are contrastive. So when we say that, for any information unit, the unmarked structure is that with the focus on the final element, this excludes any items that are inherently given.