The operating system uses internal labels to identify direct access volumes and to store data set attributes (for example, record length and block size). The attribute information must originally come from a DD statement or from your program.
IBM standard labels have two parts: the initial volume label and header labels. The initial volume label identifies a volume and its owner; the header labels precede and follow each data set on the volume. Header labels contain system information, device-dependent information (for example, recording technique), and dataset characteristics.
direct access volumes have IBM standard labels. Each volume is identified by a volume label, which is stored on the volume. This label contains a volume serial number and the address of a volume table of contents (VTOC). The table of contents, in turn, contains a label, termed a data set control block (DSCB), for each data set stored on the volume.