All the services that are running in Oracle Apps R12 are now categorized into Service Groups from R12 version of Apps. According to these services groups different services run on single or multi-node.

Below are the name of the service group and the services associated with it.

Service Group Name
Services Running
Root Service Group
Oracle Process Manager(OPMN)
Web Entry Point Services
HTTP Server
Web Application Services
Batch Processing Services
Application TNS Listener, Concurrent Managers, Fulfillment Server
Other Service Group
Oracle Forms Services, Oracle MWA Service

As per the node configuration the  Service Group Associated with the each node might look as below.May vary as per your configuration
Node Type
Service Group Enabled
Web Node
Root Service Group, Web Entry Point Services, Web Application Services
Forms Node
Root Service Group, Web Application Services, Other Service Group
Concurrent Processing Node
Root Service Group, Batch Processing Services