truck bed vs flatbed

truck bed and flatbed both are nouns.

Word	Noun	Adjective	Verb	Adverb
truck bed	Yes	No	No	No
flatbed	Yes	No	No	No

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

truck bed: the floor or bottom of a wagon or truck or trailer
flatbed: an open truck bed or trailer with no sides; used to carry large heavy objects

truck bed (noun)	flatbed (noun)
the floor or bottom of a wagon or truck or trailer	an open truck bed or trailer with no sides; used to carry large heavy objects
	freight car without permanent sides or roof