signer vs attestant

signer and attestant both are nouns.

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

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

signer: someone who signs and is bound by a document
attestant: (law) a person who attests to the genuineness of a document or signature by adding their own signature

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

signer (noun)	attestant (noun)
someone who signs and is bound by a document	(law) a person who attests to the genuineness of a document or signature by adding their own signature
someone who can use sign language to communicate	someone who affirms or vouches for the correctness or truth or genuineness of something