None.

The WM_APP constant is used to distinguish between message values that are reserved for use by the system and values that can be used by an application to send messages within a private window class. There are five ranges of message numbers.

Message numbers in the first range (from 0 through WM_USER –1) are system-defined. Values not explicitly defined in this range are reserved for use by the system.

Message numbers in the second range (WM_USER through 0x7FFF) can be defined and used by an application to send messages within a private window class. These values cannot be used to define messages that are meaningful throughout an application, because some predefined window classes already define values in this range. For example, predefined control classes such as BUTTON, EDIT, and LISTBOX may use these values. Messages in this range should not be sent to other applications unless the applications have been designed to exchange messages and to attach the same meaning to the message numbers.

Message numbers in the third range (from 0x8000 through 0xBFFF) are available for application to use as private messages. Message in this range do not conflict with system messages.

Message numbers in the fourth range (from 0xC000 through 0xFFFF) are defined at run time when an application calls the RegisterWindowMessagefunction to retrieve a message number for a string. All applications that register the same string can use the associated message number for exchanging messages. The actual message number, however, is not a constant and cannot be assumed to be the same between different sessions.

Message numbers in the fifth range (greater than 0xFFFF) are reserved for use by the system.