You are not required to specify volume serial numbers for output tapes, unless you are extending an existing tape data set. Serial numbers are required when the processing method is EXTEND or OUTINX or it is OUTPUT with DISP=MOD. If no number is specified, the mount message directs the operator to mount a scratch tape. The system obtains the volume serial number from the VOL1 label and records it in the JFCB and the UCB.

An optional user exit can identify a specific tape volume in place of a nonspecific volume. The exit is invoked when an open or EOV routine issues a mount request for a volume with no volume serial number. For more information about the Open/EOV nonspecific tape volume mount exit, see z/OS DFSMS Using Data Sets.

If you choose to specify the volume serial number, the system compares it with the volume serial number shown in the VOL1 label. When the serial numbers match, the system resets a mount switch in the unit control block to indicate that the mounted volume is verified. The mount switch is initially set when the mount message is issued to the operator. When the volume serial numbers do not match, the system may give the operator the option of having the label rewritten with the serial number of the volume requested. This occurs if the tape is not password protected, is not date protected, and is not a checkpoint/restart volume. Otherwise, the system rejects the tape and issues another mount message. Volumes are requested for mounting in the specified order. DFSMSrmm prevents an incorrect tape from being used and the label being rewritten.

If the system-wide RACF tape protection option has been specified, RACF authorization at the UPDATE level is checked. If the tape volume is not defined to RACF, processing continues. If the tape volume is defined RACF, and the user is UPDATE authorized and PROTECT=YES has not been specified, processing also continues.

If the tape volume is defined to RACF, the user is UPDATE authorized, and PROTECT=YES has been specified, processing will be abnormally terminated. If the tape volume is defined to RACF and the user is not UPDATE authorized, processing will also be abnormally terminated. When processing is abnormally terminated, the volume will be rejected if a nonspecific request is made, or the program will be abnormally terminated if a specific request is made.