ICustomHelpViewer only provides direct support for keyword-based Help. Some Help systems (especially WinHelp) work by associating numbers (known as context IDs) with keywords in a fashion which is internal to the Help system and therefore not visible to the application. Such systems require that the application support context-based Help in which the application invokes the Help system with that context, rather than with a string, and the Help system translates the number itself. 

Applications can talk to systems requiring context-based Help by extending the object that implements ICustomHelpViewer to also implement IExtendedHelpViewer. IExtendedHelpViewer also provides support for talking to Help systems that allow you to jump directly to high-level topics instead of using keyword searches. The built-in WinHelp viewer does this for you automatically. 

IExtendedHelpViewer exposes four functions. Two of them—UnderstandsContext and DisplayHelpByContext—are used to support context-based Help; the other two—UnderstandsTopic and DisplayTopic—are used to support topics. 

When an application user presses F1, the Help Manager calls

IExtendedHelpViewer.UnderstandsContext(const ContextID: Integer;
const HelpFileName: String): Boolean

int__fastcall IExtendedHelpViewer::UnderstandsContext(const int ContextID, AnsiString HelpFileName)

and the currently activated control supports context-based, rather than keyword-based Help. As with ICustomHelpViewer.UnderstandsKeyword, the Help Manager queries all registered Help viewers iteratively. Unlike the case with ICustomHelpViewer.UnderstandsKeyword, however, if more than one viewer supports a specified context, the first registered viewer with support for a given context is invoked. 

The Help Manager calls

IExtendedHelpViewer.DisplayHelpByContext(const ContextID: Integer;
const HelpFileName: String)

void__fastcall IExtendedHelpViewer::DisplayHelpByContext(const int ContextID, AnsiString
HelpFileName)

after it has polled the registered Help viewers. 

The topic support functions work the same way:

IExtendedHelpViewer.UnderstandsTopic(const Topic: String): Boolean

bool__fastcall IExtendedHelpViewer::UnderstandsTopic(const AnsiString Topic)

is used to poll the Help viewers asking if they support a topic;

IExtendedHelpViewer.DisplayTopic(const Topic: String)

void__fastcall IExtendedHelpViewer::DisplayTopic(const AnsiString Topic)

is used to invoke the first registered viewer which reports that it is able to provide help for that topic.