Merge's Unified API is designed to provide engineers with flexibility and control over data access and transformation, offering a range of features that allow for customization and extensibility. The platform's Common Models can be extended through Field Mapping, which enables the definition of custom fields and mapping to third-party APIs, while Remote Fields allow for programmatic retrieval and writing to fields not normalized by Merge. Additionally, Remote Data provides access to raw, untouched payloads from source APIs, and Passthrough Requests enable fully authenticated, direct API calls to supported integrations. These features give engineers fine-grained control over data without compromising the simplicity of a unified API, making Merge a deeply customizable platform that combines the flexibility of direct API access with the ergonomics of a normalized integration system. By using these features, engineers can tailor Merge's integration infrastructure to their specific needs, and the company offers demos for those interested in exploring their own use cases.