common room vs sitting room

common room and sitting room both are nouns.

Word	Noun	Adjective	Verb	Adverb
common room	Yes	No	No	No
sitting room	Yes	No	No	No

As nouns, sitting room is a hypernym of common room; that is, sitting room is a word with a broader meaning than common room:

common room: a sitting room (usually at school or university)
sitting room: a room in a private house or establishment where people can sit and talk and relax

Other hypernyms of common room include front room, living-room, living room, parlor, parlour.

common room (noun)	sitting room (noun)
a sitting room (usually at school or university)	a room in a private house or establishment where people can sit and talk and relax