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.
Business rules often start as simple observations, for example "customers call toll-free numbers to place orders." During the design process they develop into more detailed expressions, for example what information a customer supplies when placing an order or how much a customer can spend based on a credit limit.
You can attach business rules to your model objects to guide and document the creation of your model. For example, the rule "an employee belongs to only one division" can help you graphically build the link between an employee and a division.
Business rules complement model graphics with information that is not easily represented graphically. For example, some rules specify physical concerns in the form of formulas and validation rules. These technical expressions do not have a graphical representation.
In the case of the PDM and OOM, you can generate business validation rules attached to domains as check parameters.
Before you create business rules, formulate your rules by asking yourself the following questions:
What business problems do I want to address?
Are there any procedures that my system must respect?
Do any specifications dictate the scope of my project?
Do any constraints limit my options?
How can each of these procedures, specifications, and constraints be described?
How can each of these descriptions be classified? Possible classifications are definitions, facts, formulas, requirements or validation rules