Fit criteria are usually derived after the requirement description is written. You derive a fit criterion by examining the requirement’s description and rationale, and determining which quantification best expresses the user’s intention for the requirement. You may sometimes find that this close examination results in changes to the requirement, but these changes are for the better and should be considered quite normal; their occurrence simply means the requirement was not properly understood in the first instance.
Link · 12. Fit Criteria and Rationale