The SQL agent is part of the NetBackup Client software installed on the Windows server that is running the SQL database. In NetBackup 7.x versions, it is installed by default. (Prior versions had a separate install of the SQL agent software, but it still was installed on the same server where the SQL database was running.)
The NetBackup client machine is the one where the SQL database software is running.
The SQLHOST-NB interface is the name of the network port on the client where the NetBackup data will be sent from the client to the media server. It has a separate IP address than the client's regular IP address. This is done to prevent backup data from taking up all the network bandwidth from the server; this is sometimes referred to as a "backup network" (meaning a network specifically for sending backup data, not to act as a backup network in case the primary network goes down), where some NetBackup clients have dedicated ethernet ports used to connect to a separate network.
