Whilst creating business requirements it is important to understand business rules. Business rules are those which crop up as constraints in the requirement gathering phase.
Business rules are basically used to control, assert authority or influence the solution such that it is in line with the overall business structure and culture.
So what makes business rules different from business requirements?
Whilst business requirements are specific to one particular project with a few exceptions business rules.
For example in a university, whilst a business rule could be that each class must have at-least one instructor, business requirements would vary based on the course taken.
Also, make sure that all business requirements comply with business rules and under no circumstance should business rules be changed to create new business requirements.