Queries related to NBU

Hi Experts,

Please help me with below questions -

1. what is exact meaning of AVRD(automatic volume recognition daemon) and when tape drives goes to AVRD mode ?

2.Difference between bpcd & vnetd ports ?

3.What is PBX ? Is it really required in NBU 7.6 domain ?

4. Is there a command to check wheather we have backup or not from client machine ?

5. what all possible reasons for restore job showing - "media is in use" status. how to proceed with this to get the restore completed on priority.

Thanks and regards,

Bharath Achar