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." See also xanle'u, xanle'ule'u, xancrcereme and saskrcereme. (x3) - "thing expressed in Sign Language." See also xance, bangu.