authorisation vs legal document

authorisation and legal document both are nouns.

Word	Noun	Adjective	Verb	Adverb
authorisation	Yes	No	No	No
legal document	Yes	No	No	No

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

authorisation: a document giving an official instruction or command
legal document: (law) a document that states some contractual relationship or grants some right

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

authorisation (noun)	legal document (noun)
the act of conferring legality or sanction or formal warrant	(law) a document that states some contractual relationship or grants some right
the power or right to give orders or make decisions
official permission or approval
a document giving an official instruction or command