/usr/share/X11/xkb/compat
Edit: /usr/share/X11/xkb/compat/caps (507B)
partial xkb_compatibility "caps_lock" {
// Keysym Caps_Lock locks the Lock modifier.
// With this definition, the keysym Caps_Lock can be used without binding
// the whole key to a real modifier. This is essential when you don't
// want to use Caps_Lock on the first level.
// This should not have any compatibility issues when used together with
// other layouts which don't utilize this capability.
interpret Caps_Lock {
action = LockMods(modifiers = Lock);
};
};