Usability is all about the user experience. It refers to how easy it is for users to understand, learn, and use the software effectively under specific conditions. High usability is crucial for user satisfaction and productivity.

Regression testing is crucial when implementing a new feature within an existing system because it helps ensure that the introduction of new features does not lead to the breaking of existing functionality. This is essential for maintaining the software's stability and quality.

The Microservices architecture style is known for developing a system as a collection of smaller independent units. In this architecture, the application is composed of loosely coupled, independently deployable services, which makes it easier to develop, test, and scale.

In white box testing, the tester needs to have knowledge of the internal code structure to construct test cases and identify issues. This type of testing involves examining the internal logic, structure, and flow of the software code. Understanding the code is essential for effective white box testing.

The Spiral model is an approach to software evolution that emphasizes incremental development and operational prototyping. This model divides the project into cycles or "spirals," with each spiral incorporating user feedback and refined prototypes. This iterative approach allows for continuous improvements and risk management throughout the software development process.

The primary aim of software verification is to identify and fix defects in the software. It involves activities like code reviews and inspections to ensure the software aligns with its specifications and design.