What characterizes an external service mashup?

The characteristic that defines an external service mashup in the context of Salesforce is that it treats Force.com as the client. This means that the external service provides data or functionality to a Salesforce application, allowing Salesforce to communicate and integrate with external systems or data sources.

In this scenario, Force.com acts as the client, consuming the external service's APIs or web services. This setup enables Salesforce applications to leverage and extend their capabilities by incorporating information from external sources, which can enhance functionality and user experience.

The other choices do not align with the concept of an external service mashup. If the service operated entirely within Salesforce, it would not be considered a mashup involving external services. Additionally, the mashup does not solely rely on internal data; rather, it actively integrates external data. Finally, the use of AJAX technologies is not a restriction; while AJAX can be utilized for asynchronous web requests in a mashup, it is not exclusively tied to this technology. The ability to use various technologies for integration underscores the flexibility inherent in external service mashups.