Halliday & Matthiessen (2014: 369):
If there is no Deictic element, the nominal group is non-specific and, within that, non-singular.* In other words, a nominal group may have no Deictic element in its structure, but this does not mean it has no value in the Deictic system — simply that the value selected is realised by a form having no Deictic in the expression.* The forms trains and some trains, as in there are (some) trains on the track, are not in fact identical. But the distinction is a more delicate one, and for the purpose of this analysis they will be treated as variant expressions of the same category.