footwear vs flats

footwear and flats both are nouns.

Word	Noun	Adjective	Verb	Adverb
footwear	Yes	No	No	No
flats	Yes	No	No	No

As nouns, flats is a hyponym of footwear; that is, flats is a word with a more specific, narrower meaning than footwear:

Other hyponyms of footwear include boot, clog, geta, patten, sabot, overshoe, shoe, carpet slipper, slipper.

footwear (noun)	flats (noun)
covering for a person's feet	footwear (shoes or slippers) with no heel (or a very low heel)
clothing worn on a person's feet