musical group vs septet

musical group and septet both are nouns.

Word	Noun	Adjective	Verb	Adverb
musical group	Yes	No	No	No
septet	Yes	No	No	No

As nouns, septet is a hyponym of musical group; that is, septet is a word with a more specific, narrower meaning than musical group:

musical group (noun)	septet (noun)
an organization of musicians who perform together	a musical composition written for seven performers
	seven people considered as a unit
	a set of seven similar things considered as a unit
	seven performers or singers who perform together
	the cardinal number that is the sum of six and one