IME Messages
The operating system sends IME window messages to the window procedure of an application when certain events occur that affect the IME windows. For example, the operating system sends the WM_IME_SETCONTEXT message to the application when a window is activated. IME-unaware applications pass these messages to the DefWindowProc function, which sends them to the corresponding default IME window. IME-aware applications either process these messages or forward them to their own IME windows.
Your application can direct an IME window to carry out a command, for example, change the position of a composition window, by using the WM_IME_CONTROL message. The IME notifies the application about changes to the composition string by using the WM_IME_COMPOSITION message, and about general changes to the status of the IME windows by sending the WM_IME_NOTIFY message.