A merge operation is similar to a rejoin operation except that it occurs when nodes that are partitioned begin communicating again.
The partition may be a true partition in that cluster resource services is still active on all nodes. However, some nodes cannot communicate with other nodes due to a communication line failure. Or, the problem may be that a node actually failed, but was not detected as a failure.
In the first case, the partitions are merged back together automatically once the communication problem is fixed. This happens when both partitions periodically try to communicate with the partitioned nodes and eventually reestablish contact with each other. In the second case, you need to change the status of the partitioned node to failed from the active node. You can then restart cluster resource services on that node from any node within the cluster.