Yesterday, I described what business processes and rules are. If your company sells its products to other businesses, it is generally important to understand the processes and rules of these business customers. Such an understanding enables you to make your marketing messages and sales processes relevant to your target market.
raised this question on Seilevel's Software Requirements Message Board. Below is a transcription of my reply.
It seems that business rules and requirements have at least two relationships.
The business rules sometimes represent the context in which we specify the requirements. I.e., the existing rules of a customer organization influence or drive the requirements.
In other cases, the requirements drive business rule design. I.e., designers specify how the user will interact with the system to fulfill the requirements.
In neither case are the business rules requirements in and of themselves, though they are integrally related. See my post on the relationship between use cases and requirements for more details.