To leverage all of the instances in a cluster, create one or more distributed transaction processing (DTP) services for each Oracle RAC instance that hosts distributed transactions. Choose one DTP service for one distributed transaction. Choose different DTP services for different distributed transactions to balance the workload among the Oracle RAC database instances. Because all of the branches of a distributed transaction are on one instance, you can leverage all of the instances to balance the load of many DTP transactions through multiple singleton services, thereby maximizing application throughput.
If you add or delete nodes from your cluster database, then you may have to identify and relocate services that you are using for DTP transactions to ensure that you maintain optimum performance levels.
Oracle Database Development Guide for more information about transaction branch management in Oracle RAC