When a physical tape volume is entered into an ATLDS or MTL, the cartridge must have an external label (readable by machine, human, or both). If the volume is assigned the private use attribute on entry into the tape library, a magnetically recorded volume serial number on the tape volume label must be present and match the volume serial number on the external label. This label is not checked on entry into the tape library, so if there is an error, it is not detected until the volume has been mounted and the label has been read.
If the volume is assigned the scratch use attribute and no volume serial number exists for it, a new volume label with a matching volume serial number is written whenever the data set is opened for output on the tape. For a scratch volume with an existing volume serial number that does not match the volume serial number on the external label, the internal label is rewritten to match the external label.