upper class vs stratum

upper class and stratum both are nouns.

Word	Noun	Adjective	Verb	Adverb
upper class	Yes	No	No	No
stratum	Yes	No	No	No

As nouns, stratum is a hypernym of upper class; that is, stratum is a word with a broader meaning than upper class:

upper class: the class occupying the highest position in the social hierarchy
stratum: people having the same social, economic, or educational status

Other hypernyms of upper class include class, social class, socio-economic class.

upper class (noun)	stratum (noun)
the class occupying the highest position in the social hierarchy	an abstract place usually conceived as having depth
	a subpopulation divided into a stratified sampling
	people having the same social, economic, or educational status
	one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)