A virtual server is a group of physical servers from which the best rated server will automatically get preferred at Job execution time. In Talend Administration Center, simply select physical servers and group them into a virtual server. Then set the execution task onto this virtual server as for any execution server, in order to automatically select the best server to execute the task onto.
Access to the Virtual servers page depend on your license. For more information, see What modules and features are available depending on your license.
For users working in cluster mode via virtual servers, note that the ranking of servers to be used for load balancing is based on indicators, whose bounds (such as free disk space limits) and weight are defined in the monitoring_client.properties file which is located in <ApplicationPath>\WEB-INF\lib\org.talend.monitoring.client-A.B.C.jar. These values can be edited according to your needs. For more information, see Configuring the indicators which determine which server to be used for load balancing.