Software Development Verification vs. Validation:
Verification determines if the product accurately represents and meets the design specifications given to the developers. A product can be developed that does not match the original specifications. This step ensures that the specifications are properly met and closely followed by the development team.
Validation determines if the product provides the necessary solution intended real-world problem. It validates whether or not the final product is what the user expected in the first place and whether or not it solve the problem it intended to solve. In large projects, it is easy to lose sight of overall goal. This exercise ensures that the main goal of the project is met.
From DITSCAP:
6.3.2. Phase 2, Verification. The Verification phase shall include activities to verify compliance of the system with previously agreed security requirements. For each life-cycle development activity, DoD Directive 5000.1 (reference (i)), there is a corresponding set of security activities, enclosure 3, that shall verify compliance with the security requirements and evaluate vulnerabilities.
6.3.3. Phase 3, Validation. The Validation phase shall include activities to evaluate the fully integrated system to validate system operation in a specified computing environment with an acceptable level of residual risk. Validation shall culminate in an approval to operate.
NOTE: DIACAP has replace DITSCAP but the definition above are still valid and applicable for the purpose of the exam.
Reference(s) used for this question: Harris, Shon (2012-10-25). CISSP All-in-One Exam Guide, 6th Edition (p. 1106). McGraw-Hill. Kindle Edition. and http://iase.disa.mil/ditscap/DITSCAP.html