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Non-functional requirement (NFR) is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors.
They are contrasted with functional requirements that define specific behavior or functions. 

Wikipedia: https://en.wikipedia.org/wiki/Non-functional_requirement

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Non-functional requirements are criteria to judge that the system performs its functions the right way. For instance, the max downloaded file size is 1 Gb, it should be downloaded within 12 seconds, etc.

Examples

• Scalability
  It tells what amount of data or number of users the system should be able to process.
  
  
• Performance
  How fast a certain system functions should be performed.
  
  
• Fault tolerance
  How fast the system should be restored in case of an unexpected issue.

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• Customers
  For instance, they can provide numbers for Scalability, Performance as well as additional Security requirements.
  
  
• Hosting providers
  For instance, they can provide Fault tolerance and Availability requirements that have already been agreed upon with customers as part of SLA.
  They can also help to define Supportability requirements.
  
  
• Dev teams and Solution architects
  For instance, they can provide Maintainability and Testability requirements. POs can also consult with them for Security requirements. 

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