The scope of needs is all over the map. They can express high level business rules or mandated detail design constraints such as "We will use parts from vendor xxx." All requirements are reconciled to needs.
Referring to the Needs is how you know if the requirements are right. This is one of the critical validations of the system for suitability. If there are requirements that are not reconcilable to any needs then they should be considered for deferral. Needs are translated into formal requirements as well defined features or functional role requirements.