'Preferred server' is only applicable to passive copy backup.
See the following in NBU for Exchange Admin Guide:
Configuring a preferred server list for an Exchange 2010 Database Availability Group (DAG)
You can create preferred server configurations for an Exchange 2010 Database
Availability Groups (DAG). The preferred server list is a collection of one or more
servers in the DAG that you select as preferred backup sources. Preferred server
configurations take priority as backup sources in instances where database copies
are replicated between multiple servers. The preferred server list is required for
Passive copy only. It is ignored for Active copy only and is optional for Passive
copy and if not available the active copy.
You can let NetBackup choose the best server from which to back up the replicated
database copies or you can designate a preferred server list. Designating a
preferred server list gives you more control over your backup jobs. For example,
you can configure a list of preferred servers that are local to avoid having to back
up replicated data over your WAN. You can arrange the servers in order of
preference. Or you may have one node of a DAG that contains passive copies for
all or most of your databases and that is also a fast media server. Add only this
server to your preferred list to make the backup more efficient.
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