signer vs cosignatory

signer and cosignatory both are nouns.

Word	Noun	Adjective	Verb	Adverb
signer	Yes	No	No	No
cosignatory	Yes	Yes	No	No

As nouns, cosignatory is a hyponym of signer; that is, cosignatory is a word with a more specific, narrower meaning than signer:

signer: someone who signs and is bound by a document
cosignatory: one of two or more signers of the same document (as a treaty or declaration)

Other hyponyms of signer include cosigner, cosigner, abhorrer, endorser, indorser, attestant, attestator, attestor, witness.

signer (noun)	cosignatory (noun)
someone who signs and is bound by a document	one of two or more signers of the same document (as a treaty or declaration)
someone who can use sign language to communicate

signer (adjective)	cosignatory (adjective)
	signing jointly with others