sofa bed vs day bed

sofa bed and day bed both are nouns.

Word	Noun	Adjective	Verb	Adverb
sofa bed	Yes	No	No	No
day bed	Yes	No	No	No

As nouns, day bed is a hyponym of sofa bed; that is, day bed is a word with a more specific, narrower meaning than sofa bed:

sofa bed: a sofa that can be converted into a bed
day bed: convertible consisting of an upholstered couch that can be converted into a double bed

Other hyponyms of sofa bed include davenport, studio couch.

sofa bed (noun)	day bed (noun)
a sofa that can be converted into a bed	convertible consisting of an upholstered couch that can be converted into a double bed