Sometimes it desirable to access WiX properties defined in the extension modules. PropertyRef is designed for such cases. You can use it as an ordinary WiX property but you do not need to set it up (e.g. define initial value) as it is already done in the corresponding extension module.
In a way PropertyRef is similar to C++ #include as it makes possible to access some entities defined in the external modules.
PropertyRef inherits from Property because of their logical relationship and usability pattern, not because of any "Parent/Child" connection.