The object base contains KnowledgeWorks CLOS objects (including relational database objects) and KnowledgeWorks structures. KnowledgeWorks CLOS objects can be treated as ordinary CLOS objects and may be manipulated directly from Lisp. KnowledgeWorks relational database objects may transparently retrieve their slot values from a relational database using the LispWorks object-oriented relational database interface.
KnowledgeWorks structures are more efficient but reduced functionality CLOS objects similar in spirit to Lisp structures. Values in the slots of these objects should not be destructively modified unless these values are themselves KnowledgeWorks objects. This is because the rule interpreter keeps track of the changes to the slots, and a destructive operation is likely to bypass this process.
KnowledgeWorks and Prolog User Guide (Unix version) - 6 Dec 2011