flax vs plant fiber

flax and plant fiber both are nouns.

Word	Noun	Adjective	Verb	Adverb
flax	Yes	No	No	No
plant fiber	Yes	No	No	No

As nouns, plant fiber is a hypernym of flax; that is, plant fiber is a word with a broader meaning than flax:

flax: fiber of the flax plant that is made into thread and woven into linen fabric
plant fiber: fiber derived from plants

Other hypernyms of flax include plant fibre.

flax (noun)	plant fiber (noun)
plant of the genus Linum that is cultivated for its seeds and for the fibers of its stem	fiber derived from plants
fiber of the flax plant that is made into thread and woven into linen fabric