Relationship of the NUMBER OF BACKUP VERSIONS and RETAIN DAYS attributes

Consider a management class that specifies:

        NUMBER OF BACKUP VERSIONS (DATA SET EXISTS)        = 4
        NUMBER OF BACKUP VERSIONS (DATA SET DELETED)       = 2
        RETAIN DAYS EXTRA BACKUP VERSIONS                  = 20
        RETAIN DAYS ONLY BACKUP VERSION (DATA SET DELETED) = 60

Now consider a single data set associated with this management class:

As long as no backup version becomes more than 20 days old and the data set exists, no more than four backup versions are retained because of the action of NUMBER OF BACKUP VERSIONS (DATA SET EXISTS). Backup versions in excess of four are deleted even though they may not yet be 20 days old.
If backup versions are no longer made (for example, because the data set is not being changed), the next processing of the EXPIREBV command deletes any backup version that is more than 20 days old except for the most recent backup version. (Remember, DFSMShsm always requires you to take specific action to delete the last backup version of an existing data set.) The deletion of the oldest backup version is controlled by RETAIN DAYS EXTRA BACKUP VERSIONS. EXPIREBV treats the newest EXTRA backup copy as if it had been created on the same day as the newest BACKUP VERSION. Therefore, the newest EXTRA backup copy is not deleted until the number of days specified in RETAIN DAYS EXTRA BACKUP VERSIONS has passed, starting from the creation date of the newest backup copy.
Assume that the data set is deleted while four backup versions exist that are less than 20 days old:
The next processing of the EXPIREBV command deletes the two oldest backup versions because of the action of NUMBER OF BACKUP VERSIONS (DATA SET DELETED). Deleting the data set caused a change in the controlling attribute.
Since there are only two backup versions left, the oldest backup version is also the newest EXTRA backup version. EXPIREBV treats the newest EXTRA backup copy as if it had been created on the same day as the newest BACKUP VERSION. Therefore, the newest EXTRA backup copy is not deleted until the number of days specified in RETAIN DAYS EXTRA BACKUP VERSIONS has passed, starting from the creation date of the newest backup copy.
When 60 days have passed since a DFSMShsm EXPIREBV command has detected that the data set was deleted, the next processing of the EXPIREBV command deletes the last backup version through the action of RETAIN DAYS ONLY BACKUP VERSION (DATA SET DELETED).