ABCs of z/OS System Programming Volume 3 - IBM Redbooks

For data sets larger than 58 K tracks, DFSMShsm allocates a large format sequential data set
for the backup copy. Basic or large format sequential data sets will prefer OVERFLOW or
NOOVERFLOW volumes based on the SETSYS ML1OVERFLOW(DATASETSIZE(dssize))
value.
If there is not enough free space on the NOOVERFLOW or OVERFLOW volume for a
particular backup copy, DFSMShsm tries to create the backup on a OVERFLOW or
NOOVERFLOW volume respectively. If the data set is too large to fit on a single ML1 volume
(OVERFLOW or NOOVERFLOW), then the migration or backup fails.
Important: In processing these commands, DFSMShsm first checks the management
class for the data set to determine the value of the ADMIN OR USER COMMAND
BACKUP attribute.
If the value of the attribute is BOTH, a DFSMShsm-authorized user can use either of the
commands, and a non-DFSMShsm-authorized user can use the HBACKDS command to
back up the data set.
If the value of the attribute is ADMIN, a DFSMShsm-authorized user can use either of the
commands to back up the data set, but a non-DFSMShsm-authorized user cannot back up
the data set. If the value of the attribute is NONE, the command backup cannot be done.
218 ABCs of z/OS System Programming Volume 3