Any method (use case) may have an unlimited number of non-functional requirements (NFRs). They are sometimes called “qualities of service”. Wikipedia has a list of possible NFRs.
Requs allows NFRs be related only to methods (functional requirements). In other words, you can’t say that your entire product should be “scalable”. Or that it has to be multi-language. Or that its mean time between failure is more than 24 hours.
You should always relate such non-functional attributes to specific functionalities. There is a reason behind this restriction - we want NFRs to be testable. If they are testable - there is a tester. If there is a tester - there is some functionality. If there is a functionality - it has to be documented as a method (use case).
The syntax is simple:
UC3.2/MTBF must "be 5 minutes on a standard equipment".
UC7/PERF must "be less than 500 msec per request".
UC8/UI must "speak English and Spanish".
UC9.5.5/PERF must be "less than 100 milliseconds per HTTP request".In the future vesions of Requs we may have more formal syntax for NFRs.