Halliday & Matthiessen (1999: 531):
In ellipsis, some features which are present in the semantic construction of the clause (or other unit) are not realised explicitly in the wording, which cannot then be interpreted unless these features are retrieved from elsewhere. Here it is not the meaning that is being referred to; it is the wording that is being retrieved, usually from the immediately preceding clause (whereas reference can span considerable distances in the text). Ellipsis is particularly characteristic of dialogue, especially adjacency pairs such as question and answer. Sometimes in English a substitute element is inserted as a placeholder; e.g. ones in Which lanes are those? — The northbound ones.