A one-byte in which you specify the storage key for the storage in which the message blocks are returned. The system ignores this field unless you have indicated, by setting bit S99EMKEY in the field S99EOPTS, that you will be specifying a storage key. If you do not supply a storage key, the system uses the same key as the caller's TCB. The valid keys are 0-15, and they must be specified in binary. An unauthorized program cannot request a storage key that is different from its own key.