There exists a set of traits for effective requirements. According to the IIBA effective requirements have the following traits.
Cohesive: The requirement defines a single aspect of the change being implemented.
Complete: The requirement specifies everything that is required to understand the requirement. All requirements together cover all aspects of the change being implemented.
Consistent: The requirement does not contradict another requirement.
Consistent: The requirement does not contradict another requirement.
Modifiable: Requirements are organized to allow requirements and related requirements to be easily modified so that all requirements stay valid.
Correct: The requirement fulfills the business needs the requirement was defined for.
Feasible: The requirement can be implemented based on the constraints imposed by the engagement.
Unambiguous: The requirement is defined clearly resulting in only one interpretation of the requirement by all readers.
Verifiable: A method exists for the requirement that will show that the requirement is met once it is implemented as part of the change.
As can be seen from the list, the traits pertain to both individual requirements and to the requirements as a whole.
Looking at the whole set of requirements is important to ensure that a robust solution has been defined for the change being contemplated.
It is easier to evaluate a single requirement than it is to evaluate if a set of requirements is completed and describes an effective overall solution to the initial business needs.