Voice Command Testing primarily focuses on ensuring that an application or website can be navigated and operated using voice commands. As voice-activated devices and technologies are becoming more prevalent, it's essential for web content and applications to be compatible with such methods to aid users with specific disabilities.

The "Defect Removal Efficiency" metric quantifies the effectiveness of the testing process in identifying and rectifying defects. A higher percentage indicates that the testing process was successful in catching and resolving a large proportion of defects during testing.

When a defect is first identified and logged, it is in the "Open" state. This means that the defect is now recognized and needs investigation or action. It has yet to undergo the various phases of the defect life cycle where its validity and resolution will be determined.

Incremental Integration Testing is a process where individual units or components of a software are integrated gradually to ensure that interface defects are detected and corrected. It helps in detecting issues that can arise during component interaction.

Adapting to changes in the application under test is a challenge as any modifications in the application's UI or functionality can break existing automated test scripts. This requires regular updates to scripts, making maintenance resource-intensive.

While Load Time, Response Time, and Throughput are common metrics used in performance testing to measure various aspects of the system's performance, "Number of Defects" is not a performance metric. It is typically associated with functional testing and bug tracking.

Error Guessing is a testing technique where testers rely on their past experiences and knowledge of system failures to identify potential defects. Instead of using a systematic approach, testers use their intuition to "guess" where defects might be present and construct tests based on these hunches, drawing from previous issues they have encountered.

Load testing focuses on understanding the behavior of the system under a specific load or request volume. Metrics like latency (response time) and throughput (number of requests processed per unit time) are crucial.

Usability Testing focuses on a user's experience with the application. It ensures that the product is easy to use and provides a good user experience. Issues like button placements, text alignment, and other design-related aspects that might hamper the user's experience or make the application difficult to navigate are typically identified in Usability Testing.

During Test Planning, the Test Plan is the key document that captures all aspects of testing, including the identification and assignment of roles and responsibilities, resources, schedules, and other relevant details to guide the testing efforts throughout the cycle.