The clf relation is used in noun phrases where a cardinal numeral (1,3), an ordinal numeral (2), or a determiner (4) modifying the head noun is accompanied by a classifier. The classifier is attached to the numeral if a numeral is present, or else to the determiner. Note that ordinal numerals are tagged ADJ and labeled amod in relation to the noun; they are the only words tagged ADJ that can be accompanied by a classifier.
In other constructions involving classifiers, they are annotated with other syntax relations. First, in Mandarin a classifier may also be considered to function as an indefinite determiner when a noun phrase consisting of a bare classifier accompanying a noun is in post-verbal position, as in (4).
Second, if a noun phrase does not have a regular noun, then the classifier is promoted to head of the noun phrase and any numeral or determiner would depend on the classifier, such as in (5) and (6).
If there is a genitive 的 de between the classifier and the noun, then the classifier is also treated as a head noun for the numeral, and it is labeled as an nmod dependent of the noun.