How to find the "Identifier" value for an Xorg conf "Device" graphics card section?

Using X :2 -configure, I found out Xorg was using Card0 as the device name to configure the graphics driver:

Section "Device"
    Identifier "Card0"
    Driver     "radeon"
    Option ....
EndSection

I also tried using Radeon as the Identifier and that worked too. Where does the value of Card0 come from? In other words, is that a default value for the first graphics card found? And why does Radeon also work in its place?

The Identifier is a name that you give the Device. It is not evaluated by Xorg, it is only used when you want to refer to that Device in any other Section, like Monitor or Screen. For Devices that have no section in xorg.conf, Xorg self-assigns names, as you found out, starting with Card0. If you had two graphics cards, it would use Card1 for the second one.