String S is passed from new instance. By default parameter S is equal to first command line parameter of new instance. It may be defined using GetInstInfo event. 

Use Accept parameter to allow running a new application instance. Set Accept to False to allow running a new application instance, set Accept to True to disable running a new application instance. 

Accept means that existing instance of application accepts information from new application instance, for example, file to be opened, so new application instance may be closed.

TecSingleInstance Events