First of all, thanks for the question,
We have a few customers who have deployed Data Domain Archiver with NetWorker 7.6SP2. Data Domain Archiver has two tiers of storage: An Active tier for operational recovery/DR and a highly scalable archive tier for long-term retention. The tiering of data between the active and archive tiers is based on data movement policies defined on the Data Domain Interface. From the perspective of NetWorker, DD Archiver looks like a high capacity Data Domain system and all data, whether it's in active or archive tier, is visible to NetWorker and managed in its media catalog. You would get the benefits of DD Boost, distribute the deduplication process to the Storage Node and improve performance, just like any other DD system. Also, other benefits include Data Domain's Data Invulnerability Architecture that performs end-to-end verification and periodic scrubbing of data that preserves data integrity.
So, one example of how you could leverage DD Archiver is: let's say, your backup policy is to retain weekly fulls and daily incrementals for 60 days for operational recovery/DR and you want to retain your monthly fulls for 7 years for long-term retention (for compliance needs etc.). All backups would land in the active tier and stay there for your operational recovery. With data movement policy, only the monthly fulls can be moved to the archive tier for 7 years. From a business policy perspective, this is the data that would have moved to tape and be retained for 7 years.
Also, note that, in addition to backup data, you can use Data Domain Archiver as an archiving target leveraging applications like EMC Source One, Symantec EV, EMC CTA (Cloud Tiering Appliance), F5 ARX etc. Check Data Domain's archiving application compatibility matrices for the list of all supported applications.
For more information: http://www.emc.com/products/detail/hardware/data-domain-archiver.htm. The DD Archiver white paper gives a very good review of the product and the deployment use cases.