A requirement is a description of what the system must implement to meet the needs of users or the business. A business rule is an established rule or constraint that defines how the organization should operate, regardless of any specific system. Requirements may reference business rules to ensure their enforcement in an IT solution, but business rules exist independently of the system. For example, a requirement might state: "The system must verify the client's age," while a business rule would be: "Contracts may only be issued to persons over 18 years old."