upper crust vs social class

upper crust and social class both are nouns.

Word	Noun	Adjective	Verb	Adverb
upper crust	Yes	No	No	No
social class	Yes	No	No	No

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

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

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

upper crust (noun)	social class (noun)
the class occupying the highest position in the social hierarchy	people having the same social, economic, or educational status