Quoting from ClusterXL admin guide:
The role of each Cluster Member is chosen according to its cluster state and Cluster Member priority. Cluster Member priorities correspond to the order, in which they appear in the Cluster Members page of the cluster object in SmartConsole. The top-most Cluster Member has the highest priority. You can modify this ranking at any time (requires policy installation and causes a failover).
In addition to its role as a Firewall, the Active Cluster Member is also responsible for informing the Standby Cluster Members of any changes to its cluster state and kernel tables. This keeps the peer Cluster Members up-to-date with the current traffic passing through the cluster.