Generally, you do not need to use this method as it is called by the RuleSetInitializationController when its activated in the main window. The only scenario in which you may need to use the method is checking validation rules before the main window is displayed - when a user has not yet logged into the application. In this case, override your Module's ModuleBase.Setup method and handle the XafApplication.SetupComplete event. In the event handler, access the Validation Module and call the InitializeRuleSet method. To see an example of such InitializeRuleSet method usage, refer to the Non-Persistent Objects Validation help topic.