upper crust vs stratum

upper crust and stratum both are nouns.

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

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

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

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

upper crust (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)