In one of his posts on this website, Mr. Goldsmith says the following about nonfunctional requirements: "Most nonfunctional requirements are real business requirements that exist within the business situation and must be met by any product, system or software". In this sense, isn't it wrong to say "nonfunctional requirements are […] at the System level in the requirements hierarchy" as it is stated on here on your site since they also occur on a higher level? It would be great if you could clarify this. Many thanks in advance Frank Korspert