The one interrogative that requirements should answer is what. What defines the characteristics, attributes, and features of the requirement. What describes the desired outcome or result of the requirement. What specifies the functional or non-functional aspects of the requirement. What helps to identify and differentiate the requirement from other requirements. What also helps to verify and validate the requirement by testing whether it meets the acceptance criteria.

The other interrogatives, when, why, and how, are not essential for identifying the requirement, but they may provide additional information or context for the requirement. When defines the timing, frequency, or sequence of the requirement. Why defines the rationale, purpose, or benefit of the requirement. How defines the method, process, or technique of implementing the requirement. References:

BABOK® Guide, p. 36, 38, 40

CBAP / CCBA Certified Business Analysis Study Guide, p. 154, 156

Certified Business Analysis Professional™ (CBAP®) | Coursera, Course 2, Week 1, Video: “Specify and Model Requirements”