Explanation/Reference:
One server instance in a cluster that contains migratable servers acts as the cluster master and orchestrates the process of automatic server migration, in the event of failure.
Any Managed Server in a cluster can serve as the cluster master, whether it hosts pinned services or not.
Note: Any server instance in the cluster can serve as the cluster master. When you start a cluster that contains migratable servers, the first server to join the cluster becomes the cluster master and starts up the cluster manager service. If a cluster does not include at least one migratable server, it does not require a cluster master, and the cluster master service does not start up.
Reference: https://docs.oracle.com/cd/E12839_01/web.1111/e13709/migration.htm#CLUST285