_impl and _fieldImpl in PCDataImpl class, are these 2 fields ever used ?

[catalina wei]

Hi all,
In order to develop a DataCache plugins of our own, we need to understand
the 2 fields in the PCDataImpl.java, _impl and _fieldImpl:
    private Object _impl = null;
    private Object[] _fieldImpl = null;

Under the debugger with the DataCache enabled, I have not seen these 2
fields  ever set.

Could either Patrick or Abe ( the authors of PCDataImpl.java) help us
understand what these 2 fields are for, and how they are used in DataCache ?

Thank you.

Catalina