class Aves vs order Gaviiformes

class Aves and order Gaviiformes both are nouns.

Word	Noun	Adjective	Verb	Adverb
class Aves	Yes	No	No	No
order Gaviiformes	Yes	No	No	No

class Aves (noun)	order Gaviiformes (noun)
(ornithology) the class of birds	large aquatic birds: loons and some extinct forms