Rules constrain the sending of cost-type business process records, and affect making changes to Cost and Funding sheets. That is, they give some control over costs by helping you to enforce cost constraints. For example, you could create a rule that prevents invoices from exceeding a contract budget.
As the administrator, you can create rules and rule templates. Rules can be created at the project or shell level or company level. Rule templates can be created for project- or shell-level rules.
You will be using a formula to create a rule, much as you use formulas to create field values.