The AT+XNONCE command returns a random string that was generated at boot time. This string is used together with some other device specific identifiers as a base to let Apple generate a certificate, similar to SHSH, to allow installation of baseband firmware. The baseband checks the certificate and allows or denies installation of its firmware.
This string can be obtained by using the MobileDevice Library to call for AMDeviceCopyValue on "BasebandNonce".
References / More infos
at+xnonce? +XNONCE: "10552274670825F9892C22752D9509378F2154CA" OK