XDeviceInfo.use values that aren't in the XInputExtension spec

Question

The XInput specification defines three use values for the XDeviceInfo structure. They are:

• IsXKeyboard
• IsXPointer
• IsXExtensionDevice

These roughly equate to (if I'm reading the XInput spec correctly):

• Device is being used for core keyboard events
• Device is being used for core pointer events
• Device is available as an extension device

My X server (1.5.3) reports 4 devices

1. A "Virtual Core Keyboard" with use IsXKeyboard
2. A "Virtual Core Pointer" with use IsXPointer
3. A "Mouse0" with use IsXExtensionPointer
4. A "Keyboard0" with use IsXExtensionKeyboard

It is the last two that I am interested in. Those are the actual devices in my xorg.conf file.

My best guess is that the IsXExtensionKeyboard and IsXExtensionPointer values mean "This device is being used to send core events, but you can use it as an extension device if you want to". Am I on the right track with that? What other information about those values (and any other new features of XInput that might not be in the spec) is available?