ABAP OO modeling

Hello everyone,

I want to create an UML for the topic invoices and documents. My question is it, how to model it? I have the class invoice with the attribute structure RBKP(table). This class inherite from the class document. So what happens when I make an upcast from the object invoice to the object document? What happens with the attribute structure RBKP? Some fields of this attribute structure are general and can be used in the object document like "time of recording" or "document date". The problem is, its a part of this structure RBKP in the object invoice and cant be cast to the object document.

Thank you for help!