Is there any documentation for udev builtins?

The man page for udev mentions in several places that certain rules options can be used to invoke 'builtin' commands, which are apparently built in to the udev program itself. However, I haven't been able to find any reference documentation that clearly explains what udev builtins are available; what they do and how they are used. I have searched the web without much success. Does anyone know if there is a reference anywhere that provides details about these builtin commands?