From the ClientPak for Unix manual:
The NetWorker client software provides recover and on-demand backup functionality, and it communicates with the NetWorker server. You install the NetWorker client software on all computers that you want to back up to the NetWorker server.
NetWorker clients do not have to be of the same operating system as the NetWorker server. However, to back up clients of different operating systems, your NetWorker server must have its ClientPak implemented for permanent use. For example, the NetWorker server for Solaris™ is allowed to back up NetWorker clients on the Solaris operating system, but if you also want to back up NetWorker clients on a different operating system, then you must purchase and implement the ClientPak authorization code.