Each time that DFSMShsm makes a backup of a data set, the copy is known as a version. (Depending on the record length that is used to define the BCDS, DFSMShsm can keep up to 29, or up to 100, versions of any data set.) The number of backup versions determines how far back you can go to recover a data set. You can specify the number of backup versions to keep by using the two NUMBER OF BACKUP VERSIONS attributes in the management class.
The NUMBER OF BACKUP VERSIONS (DATA SET EXISTS) attribute controls the maximum number of backup versions you accumulate for data sets that are currently on either a level 0 volume or a migration volume. You will most likely want to keep multiple backup versions of these data sets. After you have as many versions of a data set as you specify with this attribute, each new backup version that is made of that data set causes the oldest backup version that exists to be discarded. The default for NUMBER OF BACKUP VERSIONS (DATA SET EXISTS) is 2.
For more information about how record length affects the number of backup versions, see z/OS DFSMShsm Implementation and Customization Guide.