Brian Pate of SoftwareCPR suggest that a good rule of thumb is: If differences in the final product, produced by two different development groups using the same specification element, resulted in unacceptable differences in safety or efficacy then it would likely be a “requirement”. Otherwise it is most likely to be a design specification.

This can be used as a good heuristic(especially for distinguishing software requriements fromsoftware design specifications) but with care from an overall device compliance perspective. A medical device may have features unrelated to safety and effectiveness that are still considered design inputs and may also show up in device labeling and instructions. Such features could still be considered requirement from the device perspective.