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It is just a piece of software that allows the NetBackup daemons / processes to communicate with another piece of software - in the case you refer to it is PDDO but could be a DataDomain or Quantum DXi type device
It is a device driver if you like - it sits between the two to aloow them to work.
You dont have a choice - it is not a seperate installation it is installed as part of NetBackup software for PDDO these days (other vendors do have an API that does need to be installed)
Without it you could not communicate and therefore not back up to PDDO