Halliday & Matthiessen (1999: 184-5, 211):
Semantically, Classifiers are qualities of the 'class' type (cf. Table 5(4)): they are like things and may be derived from things, but unlike things they do not have independent existence — they cannot be established in referential space and re-identified in running discourse. So for example although a 'passenger' is undoubtedly a thing, in a passenger train, where passenger functions as Classifier, it is being construed as a quality; hence it cannot be picked up by anaphoric reference — we cannot say this is a passenger train; they must have valid tickets. (Contrast this train is for passengers; they must have valid tickets, where passengers is functioning as Thing.) Grammatically, Classifiers are realised by 'substantives' or by 'adjectives', and this indeterminacy in grammatical class is symbolic of their status as qualities which are like things.
