Non-functional requirements are not directly related to the characteristics or Features of an application, but rather to other aspects of design and development: they define how the application should function in various circumstances, particularly under load or error, as well as how it will evolve according to different levels of use.

For example, in the case of a real-time critical application requiring intervention in the shortest possible time, or in the case of an application likely to be used simultaneously by thousands of clients, it is important to specify this. This is done in the context of a non-functional requirement formulated in this way:

• The system must be able to handle 100,000 simultaneous clients with a response time of less than 2 seconds per user.
• The site must load in 3 seconds when the number of simultaneous users is > 10,000.
• Emails must be sent with a latency not exceeding 1 minute.