Halliday & Matthiessen (2014: 116):
In the idealised form each information unit consists of a Given element accompanied by a New element. But there are two conditions of departure from this principle. One is that discourse has to start somewhere, so there can be discourse-initiating units consisting of a New element only. The other is that by its nature the Given is likely to be phoric — referring to something already present in the verbal or non-verbal context; and one way of achieving phoricity is through ellipsis, a grammatical form in which certain features are not realised in the structure. Structurally, therefore, we shall say that an information unit consists of an obligatory New element plus an optional Given. The way this structure is realised is essentially ‘natural’ (non-arbitrary), in two respects:(i) the New is marked by prominence;
(ii) the Given typically precedes the New.