The project scope statement defines the project's scope, major deliverables, assumptions, and constraints. It provides a detailed description of the project's objectives, deliverables, acceptance criteria, and the product scope. In the context of a software development project, the project scope statement would outline the functionalities and features that are within the project's scope.
The stakeholder register is a document that includes information about project stakeholders, their interests, expectations, and their level of influence on the project. While the stakeholder register is essential for understanding stakeholder communication needs and engagement levels, it might not be the most appropriate tool to prevent the situation described in the scenario.
In summary, the project scope statement is a more direct and relevant tool for preventing the situation where a key functionality is missed during testing.