A one-byte field in which you specify the subpool containing the message blocks returned to the caller. The system ignores this field unless you have indicated, by setting bit S99EMSUB in the field S99EOPTS, that you will be specifying a storage subpool. If you do not indicate a subpool, the system uses a default subpool of 0. The valid subpools are subpools 0-255, and they must be specified in binary. An unauthorized program cannot request a system subpool.