Hello,
When you enable the load balancing feature and provide and IP of the load balance IP address what occurs is NAC assumes that there is an external load balancer at the IP address provided and on enforce attempts to write this IP address as the RADIUS server to all switches in the switches tab.
Every switch is then also written into every's NAC's "clients.conf" file.
The NACs themselves do not perform any type of load balancing. As Erik has mentioned typically the lad balance IP provided in the NAC configuration is the VIP for the LSNAT configuration on the S series.
Thanks
-Ryan