A dump hierarchy is a logical structure that helps define the relationship between full and incremental dumps. As mentioned previously, an incremental dump includes only the files that changed since the fileset was last dumped; when creating an incremental dump, the Backup System uses a previous dump, known as the dump's parent, to serve as a reference point on which to base the incremental dump.
A dump set is the product of dumping a fileset family at a certain dump level, which is an entry in the dump hierarchy. The dump set is a collection of data from filesets dumped at the same time and in the same manner (fully or incrementally). To create a dump set, you specify (with the bak dump command) both the fileset family and the dump level. The Backup System keeps all of the data in a dump set together on a tape (or set of tapes, if the dump set is too large to fit on a single tape). The name of the dump set consists of the name of the fileset family and the last component of the name of the dump level joined by a period (fileset_family_name.dump_level).
Each dump level can be associated with an expiration date that specifies when a tape containing data from that dump level can be overwritten. The expiration date is transferred to any backup tape that contains a dump made at that level. When dumping to tape, the system checks the tape for an expiration date. If the tape's expiration date has not expired (if it is in the future), the system does not overwrite the tape; if no expiration date is defined for the tape or if the tape's expiration date has expired (if it is in the past), the system overwrites the tape, but only with a dump set of the same name.