Where does the challenge value go in the Registration JWT?

[tblachowicz]

In the high-level overview diagram explains that the Server returns the random challange value in Sec-Session-Registration response header and then the Browser generates the signed Registration JWT that should contain the challange value provided by the Server. It's not clear to me where does the challange value go in the registration JWT. The description of the JWT in Start Session section mentions that jti is a nonce. Please clarify if that is meant to be the challange value provided by the server. If so, it'd be good to clarify that in the Explainer to avoid confusion.

[bc-pi]

#43 is similar/duplicative FWIW