If the IME does not have the capability of ImeSetCompositionString, it does not specify any SCS capability in IMEINFO structure. If the IME can handle ImeSetCompositionString, it sets the SCS_COMPSTR bit. If the IME can generate the reading string from the composition string, it may set the SCS_CAP_MAKEREAD bit.
If the IME has the SCS_CAP_COMPSTR capability, ImeSetCompositionString will be called. The IME takes the new composition string from the application and generates the WM_IME_COMPOSITION message.
If the IME has SCS_CAP_MAKEREAD capability, it can make the reading string from the composition string.