VCL applications support displaying Help using an object-based mechanism that allows Help requests to be passed on to one of multiple external Help viewers. To support this, an application must include a class that implements the ICustomHelpViewer interface (and, optionally, one of several interfaces descended from it), and registers itself with the global Help Manager. 

VCL applications provide an instance of TWinHelpViewer, which implements all of these interfaces and provides a link between applications and WinHelp. 

The Help Manager maintains a list of registered viewers and passes requests to them in a two-phase process: it first asks each viewer if it can provide support for a particular Help keyword or context, and then it passes the Help request on to the viewer which says it can provide such support. 

If more than one viewer supports the keyword, as would be the case in an application that had registered viewers for both WinHelp and HyperHelp on Windows, the Help Manager can display a selection box through which the user of the application can determine which Help viewer to invoke. Otherwise, it displays the first responding Help system encountered.