Object-Oriented Terminology – What is a Polymorphic Method?

I say that polymorphism is a generic trait, that can be implemented several ways:

• class based inheritance.
• prototype based objects (with or without inheritance)
• duck typing
• interface compliance (as done by Go's interfaces and implicitly on C++ templates)

each of them allows the programmer to use a single code with different types, so all embody the concept of polymorphism.

In this case the base class should know nothing of SectionA. The derived class should implement the extra properties that that type needs.

For certain operations where another class needs to pull information out regardless of the type of document you will want that method on the base class ideally virtual with a basic implementation and allow derived classes to override it if needed (eg ToPlainText which would just output all the sections of a document would be on Document, FancyDocument can override the implementation to also output SectionA).

For instances where another class doesn't care about the type of document but does care if it has certain properties use interfaces. IDocument would have all the common sections and Document would implement it. IDocumentWithSectionA would inheerit IDocument and FancyDocument would implement that. This then lets you derive another NeitherFancyNorNotFancyDocument that has SectionA which can also implement IDocumentsWithSectionA.

Obviously You'd have more useful names than IDocumentWithSectionA but that depends on the usecase.

TL;DR use the abstract class for what should be common to all documents and some common functionality, use interfaces as the contract saying what a document has.