A file system cluster consists of one primary, and up to 31 secondaries. The primary-secondary terminology applies to one file system, not to a specific node (or hardware platform). So it is possible to have the same cluster node be primary for one shared file system, while at the same time it is secondary for another shared file system. Such distribution of file system primaryship to balance the load on a cluster is a recommended administrative policy.
See Distribute the load on a Cluster.
For CVM, a single cluster node is the master for all shared disk groups and shared volumes in the cluster.