Specifying migration of data sets to overflow volumes
You can ADDVOL migration volumes (ML1) to be OVERFLOW volumes
and they will be considered for backup or migration of data sets when
either one of the following are true:
The data you are migrating or backing up is larger than a given
size, as specified on the SETSYS ML1OVERFLOW(DATASETSIZE(dssize)) command
DFSMShsm cannot allocate enough space on a NOOVERFLOW volume by
selecting either the least used volume or the volume with the most
free space.