Halliday & Matthiessen (2014: 377):
The Classifier indicates a particular subclass of the thing in question, e.g. electric trains, passenger trains, toy trains. Sometimes the same word may function either as Epithet or as Classifier, with a difference in meaning: e.g. fast trains may mean either ‘trains that go fast’ (fast = Epithet) or ‘trains classified as expresses’ (fast = Classifier).