Which use cases are applicable for side-by-side extensibility?

Side-by-side extensibility refers to the approach in which a core application remains unchanged while new features or functionalities are added to it in a separate environment. This allows for greater flexibility and customization without disrupting the existing system.

When considering the two use cases in the question:

Creating a new UI for the existing application falls under side-by-side extensibility, as it allows developers to design a custom user interface that can interact with the core application while treating the original functionality and data as unchanged. This means that enhancements can be made to improve user experience or to meet specific business requirements without altering the underlining application itself.

The need to extend the existing application with new functions also aligns with side-by-side extensibility. Developers can add new capabilities or features that expand the application's functionality, again without modifying the original application. This could involve integrating third-party services or creating new business logic that enhances overall capability.

Thus, both use cases accurately reflect the concept of side-by-side extensibility, making the choice that includes both scenarios the correct answer.