overnight case vs suitcase

overnight case and suitcase both are nouns.

Word	Noun	Adjective	Verb	Adverb
overnight case	Yes	No	No	No
suitcase	Yes	No	No	No

As nouns, suitcase is a hypernym of overnight case; that is, suitcase is a word with a broader meaning than overnight case:

overnight case: a small traveling bag to carry clothing and accessories for staying overnight
suitcase: a portable rectangular container for carrying clothes

Other hypernyms of overnight case include bag, grip, traveling bag, travelling bag.

overnight case (noun)	suitcase (noun)
a small traveling bag to carry clothing and accessories for staying overnight	a portable rectangular container for carrying clothes