Non-functional requirements are properties, or qualities, that the product must have if it is to be acceptable to its owner and operator. In some cases, non-functional requirements—these specify such properties as performance, look and feel, usability, security, and legal attributes—are critical to the product’s success, as in the following case:
Non-functional requirements are qualities the product must have.
The product shall be able to determine “friend or foe” in less than 0.25 second.
Sometimes they are requirements because they enhance the product or make people want to buy it:
The product shall provide a pleasing user experience.
Sometimes they make the product usable:
The product shall be able to be used by travelers in the...
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