The optional write-only UpdateKey Objectcan be used if the underlying system requires some key to unlock the update feature. The format and where to get the key is vendor-specific and not described in this specification. If UpdateKeyis supported, the Clientshall set the key before the installation. If the PrepareForUpdateStateMachineis used, the UpdateKeyshall be set before the Prepare Methodis called. The Servershall not keep the value for more than one update.