A business rule is a rule that your business follows. It is a written statement specifying what an information system must do or how it must be structured. It could be a government-imposed law, a customer requirement, or an internal guideline.
In the case of the Requirements Model, a business rule should be used as a super requirement, i.e. a requirement defined for requirements. For instance, a business rule could be a methodological rule for writing requirements.
Business rules are not available by default in a requirements model and must be activated before you can use them.