Sentence:  Members of higher and lower social status often cluster around this nucleus , so that Protestant figures on social class give the impression of spread over all social classes ; but this is deceptive , for the core of membership <WSD>is</WSD> concentrated in a single social and economic stratum . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)