What type of actions provides additional functionality not found in the base instance?

The choice of Custom Actions is appropriate because these actions are specifically designed to extend the capabilities of the Flow Designer beyond the standard features offered in the base instance. Custom Actions allow developers to create tailored functionalities that can meet specific business requirements that are not addressed by the existing core actions.

Through the creation of Custom Actions, developers can encapsulate complex logic or integrate with other applications and services, providing enhanced flexibility and reuse within various flows. This is beneficial for organizations that require unique process automation capabilities that cater to their specific workflows.

In contrast, Core Actions refer to the basic set of actions that come pre-defined with the ServiceNow platform, while Flow Actions are a broader category that encompasses any actions that are part of the flow design. Reusable Actions, while they allow for some level of customization and efficiency, do not provide the same level of additional functionality as Custom Actions. Thus, Custom Actions stand out as the best option when looking for ways to implement unique features not available in the standard setup.