Opening an output data set
If IBM standard labels are specified, the first existing record on the output tape must be an IBM standard volume label (VOL1). At the time the data set is opened, the system checks the first existing record on the tape to determine whether the record is 80 bytes long and contains the identifier VOL1 in the first 4 bytes. The various error conditions that can occur during verification of the first record are explained in IBM standard volume label editor routines.
If the system-wide RACF tape protection option has been specified and the DD statement has specified PROTECT=YES, and the DD statement has not been previously opened for output processing, open ensures the PROTECT=YES specification is valid. Both the volume sequence number and the file sequence number must be set to 1, and a private volume must be requested. The protection indicator in the JFCB is reset so that subsequent OPENs of that DD statement for output processing do not attempt validity checking (of the PROTECT=YES specification) and defining the volume to RACF.