This read-create OCTET STRING (up to 255 bytes) is the shared secret used to authenticate and encrypt RADIUS traffic between this Virtual Router and this RADIUS server. An admin must set this to exactly match the secret configured on the RADIUS server, since any mismatch causes silent authentication failures rather than an obvious error. For example, after rotating shared secrets across the RADIUS infrastructure for a security policy update, the admin must update this key on every VR pointing at that server.