The Selenium Grid allows users to extend their testing capabilities by automatically sending overflow or different tests to Sauce Labs, a cloud-based vendor. This feature, known as the Relay Node, enables users to scale their testing while maintaining control over their existing Grid infrastructure. With the Relay Node, users can route tests to another Grid, network, or cloud vendor like Sauce Labs, making it easier to manage test execution and reduce maintenance overhead. The Relay Node also supports mobile testing with Appium, further simplifying the process of managing test infrastructure. By leveraging the Relay Node, users can build a flexible and evolving test grid that adapts to their changing needs, reducing costs and increasing efficiency.