Appends a newly created entity instance to the model.

An OdDAI object identifier of the appended entity instance.

If the specified entity has been already appended to the model, the method returns the existing entity's identifier; If the specified entity belongs to another model, the method returns a null identifier (the isNull() method of the identifier object returns true). If populated entity extent folder(s) for appended instance type and all its supertypes exist(s) within a model, the instance will be added in all these folders, so all active entity extent aggregates and iterators will become unavailable.