The update region is a subregion of a window that is deemed "invalid" or incorrect in visual terms and is in need of redrawing.

If the window has a CS_SYNCPAINT style, it is redrawn during the processing of this function and the update region should be NULL on return from this function.

If the window has a WS_CLIPCHILDREN style with part of its update region overlapping child windows with a CS_SYNCPAINT style, those children are updated before this function returns.

This function should not be called in response to a WM_PAINT request for windows of style CS_SYNCPAINT. CS_SYNCPAINT means that windows are updated synchronously when invalidated, which generates a WM_PAINT message. Thus, invalidating the window in response to a WM_PAINT message would cause another invalidate, another WM_PAINT, and so on.