The application requires exclusive access. If exclusive access is granted, no other instance of the device can obtain exclusive access to the device while it is acquired. However, nonexclusive access to the device is always permitted, even if another application has obtained exclusive access. An application that acquires the mouse or keyboard device in exclusive mode should always unacquire the devices when it receives WM_ENTERSIZEMOVE and WM_ENTERMENULOOP messages. Otherwise, the user cannot manipulate the menu or move and resize the window.
How can an instance have non exclusive access to a device when another instance is having exclusive access to it?