Product requirements document (PRD)

For hardware, a good PRD covers users and use cases, functional requirements, performance targets with numbers and test conditions, environment (temperature, water, drops), power and battery life, connectivity, target markets and certifications, target cost and volumes, and what is explicitly out of scope. Every requirement should be testable.

Requirements set the scope of engineering, the test plan and the certification path. Changing them late is the most expensive kind of change in hardware, because it can affect tooling, boards and certifications already completed.

Where it bites: vague requirements ("long battery life", "rugged") lead to disagreements at DVT, and a missing market means a missing certification.