The DPMDB is the caching database that contains all data in the DPM Backing Tree.
This database can be regenerated from the DPM Backing Tree when needed, such as in the event that the RPMDB is corrupted or tampered with.
The RPMDB can also be used to regenerate the Backing Tree in the event that the same happens with the backing tree, though this should be used sparingly as the DPM Backing Tree should be the source of truth on the system.
The Backing Tree greatly accelerates the reports that are generated by DPM, listed out in the DPM Use Cases portion of the spec.
The structure of the database definition is otherwise left undefined beyond simply mirroring the data in the Backing Tree so that the structure of the data is represented in the DPM Backing Tree portion of the specification.