When you specify the UNASSIGN subparameter with the BACKUP parameter, UNASSIGN specifies that the volume is no longer associated with the day in the backup cycle or with the set of spill volumes. The volume becomes an unassigned backup volume. All volume and data set information is kept in the BCDS and, for tape backup volumes, in the OCDS. The unassigned volume can be selected as a daily or spill backup volume.