This internal property returns a string value containing the class name of the containing object.
Every object type must implement this property.
Host objects may supply any value as a Class identifying string. They may even masquerade as one of the built-in classes, but good sense suggests that if they do, then they must obey the protocol of that built-in class in precisely the same way as if they were a real built-in object. It's probably sensible for host implementers to avoid overloading the built-in class names like that.
At edition 2 of the ECMA standard, there is no publicly accessible method to retrieve this property in a script. However, the reserved keyword values suggest that this may be offered at a later revision of the standard.