The Applications services are categorized into service groups according to the type of service provided. They were enabled and disabled as per the requirement. 
Link to earlier post Service group in R12.
In this post we are going to see the services group in R12.2 and changes when compared to previous versions.

Root Service Group: It contains Node Manager and not Oracle Process Manager (OPMN) as in previous version. Node manager services should run on all application nodes.

Web Administration Service Group: has been introduced in Release 12.2 and it contains WebLogic Administration server.  It is not supported to enable WebLogic Administration server on any other Application tier node except the node on which it was enabled during Rapid Install.

Web Entry Point Services Group: In R12.2 OPMN only manages Oracle HTTP Server and it is now part of the 

Service Group
Service Control Script
Root Service
Node Manager
Web Administration
WebLogic Admin Server
Web Entry Point Services
Oracle HTTP Server
Oracle Process Manager
Web Application Services
Batch Processing Services
Oracle TNS Listener
Concurrent Manager
Fulfillment Server
Oracle ICSM
Other Services
Forms Server
Oracle MWA Service

Parameters in Context file related to Service Groups