Halliday & Matthiessen (2014: 378):
A sequence of Classifier + Thing may be so closely bonded that it is very like a single compound noun, especially where the Thing is a noun of a fairly general class, e.g. train set (cf. chemistry set, building set). In such sequences the Classifier often carries the tonic prominence, which makes it sound like the first element in a compound noun. Noun compounding is outside the scope of the present book; but the line between a compound noun and a nominal group consisting of Classifier + Thing is very fuzzy and shifting, which is why people are often uncertain how to write such sequences, whether as one word, as two words, or joined by a hyphen (e.g. walkingstick, walking stick, walking-stick).
The relation between Classifier and Thing may be elaborating, extending or enhancing. Halliday & Matthiessen (1999: 183):
