A topmost window is a window that has the WS_EX_TOPMOST style. Topmost windows are above all non-topmost sibling windows in the Z order. You can create a topmost window directly by specifying the WS_EX_TOPMOST style when you create the window. You can also make a window a topmost window by calling the SetWindowPos function and setting the hWndInsertAfter parameter to HWND_TOPMOST.
//Notice Follow,Maybe Helpful:
A window may lose its topmost style by calling SetWindowPos and setting the hWndInsertAfter parameter to HWND_NOTOPMOST. If a window is positioned directly after a non-topmost window, then that window loses its WS_EX_TOPMOST style.