Give me the definition of a business rule
It's a brief, precise and unambiguous description of a policy, procedure or principle within a specific organisation.
They are used to define entities, attributes, relationships and constraints.
Knowing the business rules promotes the creation of an accurate data model based on how the organisation actually works and what role is played by the data within in an organisation.
Not all rules can be modeled but they can still be enforced by application software.