A message identifier is an integer-sized constant. Windows reserves the messages below 1,024 for its own use, so when you declare your own messages you should start above that level. 

The constant WM_APP represents the starting number for user-defined messages. When defining message identifiers, you should base them on WM_APP. 

Be aware that some standard Windows controls use messages in the user-defined range. These include list boxes, combo boxes, edit boxes, and command buttons. If you derive a component from one of these and want to define a new message for it, be sure to check the Messages unit to see which messages Windows already defines for that control.