X has the concept of core devices. There is exactly one core pointer and exactly one core keyboard. Before input-hotplug support, the X server would just take the first mouse and first keyboard specified in the configuration. Now there is a virtual core pointer and a virtual core keyboard. This allows X to be started with no physical devices.

All other devices, even when configured to send core events, are extension devices and treated differently. Each time a driver passes data to X, an extension event is generated. When this event is put into the event queue, the original device will be put into the EQ as well. All along the event processing the server will take into account the device the event came from. A core event on the other hand will - no matter which physical device it comes from - always appear to originate from the respective core device. GetPointerEvents() and GetKeyboardEvents() are responsible for generating xEvents from driver data. No core event will ever come from one of your other devices, they will all come from the virtual core pointer/keyboard. To the client this doesn't matter. Inside the server this is a important thing to know, as it affects grabs and other things.