N5220 appliance - OST

Can you elaborate ?

OST is a plugin to Netbackup, there are plugins from various vendors like EMC,Sepaton and Quantum that  let Netbackup utilize control the vendors hardware to Netbackup full knowledge (like replicating). 

If's a traditional Netbackup installation or  a appliance box does not make much of a difference. You can still enjoy the full power of OST is needed. 

thanks for your response.

So with NetBackup appliance (with one/two storage shelves attached) we still can add 3rd party Open Storage devices i.e. DataDomain etc.??

Yes for the 5200 series - not for the 5000 series (PDDO appliances).

Thanks - last but not least;

If I have two NetBackup 5000 appliances in 2 different DCs replicating to each other using NBU SLP; can I change the replication target on the media servers to a 3rd party appliance (i.e. Dell DR4000 deduplication)?