legal brief vs legal instrument

legal brief and legal instrument both are nouns.

Word	Noun	Adjective	Verb	Adverb
legal brief	Yes	No	No	No
legal instrument	Yes	No	No	No

As nouns, legal instrument is a hypernym of legal brief; that is, legal instrument is a word with a broader meaning than legal brief:

legal brief: a document stating the facts and points of law of a client's case
legal instrument: (law) a document that states some contractual relationship or grants some right

Other hypernyms of legal brief include instrument, legal document, official document.

legal brief (noun)	legal instrument (noun)
a document stating the facts and points of law of a client's case	(law) a document that states some contractual relationship or grants some right