Halliday & Matthiessen (1999: 197):
This syntagmatic resource serves prototypically to construe things into strict taxonomies, based on the principle of hyponymy. Thus a touring bike is a kind of bike, a reinforced touring bike is a kind of touring bike, and so on. Since touring is a Classifier, this means that it is one of a defined set: perhaps touring/racing/mountain/exercise/trail. Epithets do not assign classes, but they specify a particular dimension of taxonomic space: e.g. source European/American/Japanese...'.One type of classification is meronymic (part-whole), where the thing is classified by the whole of which it forms a part, e.g. bicycle wheel 'wheel (that forms part) of a bicycle'. These are often indeterminate in meaning: if definite, they tend to be strictly meronymic (somebody stole my bicycle wheel), whereas if indefinite, they are often classifying by type (the kids were playing with a bicycle wheel 'wheel for/ from a bicycle').