Software validation checks that the software product satisfies or fits the intended use (high-level checking), i.e., the software meets the user requirements, not as specification artifacts or as needs of those who will operate the software only; but, as the needs of all the stakeholders (such as users, operators, administrators, managers, investors, etc.).
There are two ways to perform software validation: internal and external.
Determining the degree of M&S accuracy is required because all M&S are approximations of reality, and it is usually critical to determine if the degree of approximation is acceptable for the intended use(s). In mission-critical software systems, where flawless performance is absolutely necessary, formal methods may be used to ensure the correct operation of a system.
However, often for non-mission-critical software systems, formal methods prove to be very costly A test case is a tool used in the process.
Then, it also implies the use of the Design Specification to feed the construction process.
Every time the output of a process correctly implements its input specification, the software product is one step closer to final verification.
Software verification ensures that "you built it right" and confirms that the product, as provided, fulfills the plans of the developers.