order Gaviiformes vs class Aves

order Gaviiformes and class Aves both are nouns.

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

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