• Requirements – Capture Features and Functions of a system or component.
• Constraints – Define the Non-Functional aspects of a system or component, such as restrictions on technology, resources or techniques to be used.

Also Capture a requirement at the level where it will be tested.

• If the “requirement” cannot be tested at the level you are working, then it should be captured at a lower or higher level.
  • Pay careful attention to the test aspects; capture any spinoff requirements for test stands or benches; so that when that time comes the test environment is ready.
• Capture the other decisions made at each level in the “design documentation” for that level.
  • Architecture Constraints
  • Design Constraints
  • Resource Constraints
  • When capturing these constraints, do so in the design documentation.

Read this Avionics Whitepaper: Practical Rules for DO-178C Requirements Capture