fille vs ring girl

fille and ring girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
fille	Yes	No	No	No
ring girl	Yes	No	No	No

As nouns, ring girl is a hyponym of fille; that is, ring girl is a word with a more specific, narrower meaning than fille:

fille: a young female
ring girl: a young woman who holds up cards indicating the number of the next round at prize fights

fille (noun)	ring girl (noun)
a young female	a young woman who holds up cards indicating the number of the next round at prize fights