The GoF book defines the Factory Method pattern in terms of an ICreator interface that defines a virtual function, CreateIProduct(). The virtual function CreateIProduct() returns a base class, IProduct. So, classes that derive from ICreator implement CreateIProduct() to return a subclass of IProduct. In other words, a SubclassOfCreator would create a SubclassOfProduct.
Like the Abstract Factory pattern, this enforces object cohesion – the SubclassOfCreator creates only the types that it can work with. It also allows for extensibility, because the framework is only providing the ICreator and IProduct interfaces. The client code will derive from ICreator and IProduct, which eliminates the need for client-specific behaviors in the framework’s code.