RE: How do you come up with non-functional requirement...
There are a variety of methods to elicit non-functional requirements. One method that I use is to identify upfront (before the project even starts) the main categories of non-functional requirements as well as a standard set of questions to ask for each type of non-functional requirements.
Here are some key categories of non-functional requirements: 1. GUI look and feel requirements 2. Performance requirements 3. Availability requirements 4. Security requirements 5. Capacity requirements 6. Exception Handling requirements 7. Logging requirements 8. Extensibility requirements 9. Data Retention requirements etc.
Then for each type of non-functional requirement you could develop standard questions or just as more general questions such as: "Data Retention: do you have any requirements related to how long data (or certain types of data) should be retained by the system?"
RE: How do you come up with non-functional requirement...
Functional Requirements are MUST-TO-HAVE (without these requirement the system or part of the system will not function)
Non-Functional Requirement are NICE-TO-HAVE (without these requirement the system will work, however it will help to enhance the usability and performance of the system. ex. when you have entering product code in cashier system you can do it by manual entering the product code by keyboard or use the scanner to read the code. the scanner is enhancing your usability of the system)
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