What aspect of the external system must be specified in a Connected System?

In a Connected System, specifying its base URL and authentication is essential because this information establishes the connection parameters between Appian and the external system. The base URL serves as the endpoint for API requests, allowing Appian to know where to send its requests for data or services.

Authentication is crucial for ensuring that the requests made by Appian to the external system are authorized; this may involve using credentials such as API keys, tokens, or other forms of authentication that the external system requires. By providing this information, the Connected System can successfully interact with the external service, enabling the retrieval and manipulation of data as needed.

The other aspects mentioned in the choices, such as data processing capabilities, user interface, and storage capacity, are not typically required for establishing a connected system. Instead, these elements are more relevant to the internal structure and functionality of the external system itself or how users interact with it, rather than the connection setup process. Therefore, ensuring proper specification of the base URL and authentication in a Connected System is key to enabling effective communication between Appian and the external environment.