Nonfunctional Requirements (NFRs) define system attributes such as security, reliability, performance, maintainability, scalability, and usability.
Nonfunctional Requirements serve as constraints or restrictions on the design of the system across the different backlogs.
NFRs are critical as functional for many systems. They ensure stability and effectiveness of the entire product. Failing to meet one of them can result fail to satisfy user, business, or market needs.
From Managing Requirements [FURPS]: Functionality, Usability, Reliability, Performance, and Supportability FURPS is a placeholder for nonfunctional requirements.
NFRs can be in any layer of an application.