b1 is a Signed Language used by x2=b2 to express/communicate b3 (si'o/du'u, not quote).
(x1) "Sign Language" in the sense of visually transmitting patterns using hands, arms, body, and facial expression to convey meaning. (i.e., Manual communication, body language.) (x2) "Sign Language speaker." (x3) "thing expressed in Sign Language." Cf. xance, bangu