The master node is the required, initial node in your vRealize Operations Manager cluster.

The master node performs administration for the cluster and must be online before you configure any new nodes. In addition, the master node must be online before other nodes are brought online. If the master node and replica node go offline together, bring them back online separately. Bring the master node completely online first, and then bring the replica node online. For example, if the entire cluster were offline for any reason, you would bring the master node online first.