How do I link fields from different sections?

I understand that some sections are related to each other and you can link between fields without any problems. However, some sections are not related. For instance, if I have a company address field in the customers section, I can't transfer this field to invoices section since an invoice has already contain a different field for that. You can't map those two whatsoever. I am sure there is a solution for this but I can't seem to find it. So can someone please assist me ?