Besides supporting the standard driver messages, a user-mode driver's DriverProc function often supports additional customized messages that are specific to each multimedia device type. (User-mode audio device drivers support customized messages by providing additional entry point functions.)
Although applications can send customized driver messages directly by calling SendDriverMessage, described in the Win32 SDK, typically they do not. Instead, they call functions provided by higher level multimedia APIs. These APIs in turn act as clients to the user-mode drivers and send customized messages by calling SendDriverMessage.
Customized driver messages are described in the chapters discussing the various driver types. The chapters are listed in Types of Multimedia Devices.